Actually nobody knows for sure when the Romulans split off from the Vulcans, but it must have been a very long time ago.

To summarize a long answer, the Vulcans and the Romulans should have separated several thousand years before Christ.

In the original Romulan episode "Balance of Terror" 15 December 1966, Spock says:

SPOCK: Referring to the map on your screens, you will note beyond the moving position of our vessel, a line of Earth outpost stations. Constructed on asteroids, they monitor the Neutral Zone established by treaty after the Earth-Romulan conflict a century ago.

[Sickbay]

SPOCK [OC]: As you may recall from your histories, this conflict was fought,

[Engineering]

SPOCK [OC]: By our standards today, with primitive atomic weapons and in primitive space vessels

[Bridge]

SPOCK: Which allowed no quarter, no captives. Nor was there even ship-to-ship visual communication. Therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous, and only the Romulans know what they think of Earth. The treaty, set by sub-space radio, established this Neutral Zone, entry into which by either side, would constitute an act of war. The treaty has been unbroken since that time. Captain.

Thus establishing that nobody on the Earth or Federation side knows what Romulans look like.

Later Spock manages to intercept a transmission from the Romulan ship:

SPOCK: I have a fix on it, Captain. I believe I can lock on it, get a picture of their Bridge.

KIRK: Put it on the screen.

(Up shimmers an image of a group of four humanoids around a console. One leaves his post and salutes the figure with his back to us. That figure then turns, and we see someone who looks just like - a Vulcan. Both Spock's eyebrows hit the ceiling. There's a long silence and a lot of stares.) KIRK: Decoding?

Later, in the briefing room, discussing and arguing about their options:

SPOCK: Yes, indeed we do, Mister Stiles. And if Romulans are an offshoot of my Vulcan blood, and I think this likely, then attack becomes even more imperative.

MCCOY: War is never imperative, Mister Spock.

SPOCK: It is for them, Doctor. Vulcan, like Earth, had its aggressive colonising period. Savage, even by Earth standards. And if Romulans retain this martial philosophy, then weakness is something we dare not show.

http://www.chakoteya.net/StarTrek/9.htm[1]

Spock doesn't mention any historic group of Vulcans that he thinks that the Romulans might be descended from. But he does know from Vulcan history that it is possible for a society descended from Vulcans to exist on some distant planet.

In fact, in the later episode "The Paradise Syndrome" 4 October 1968:

SPOCK: You prescribed rest, Doctor. The symbols on the obelisk are not words. They are musical notes.

MCCOY: Musical notes? You mean it's nothing but a song?

SPOCK: In a way, yes. Other cultures, among them certain Vulcan offshoots, use musical notes as words. The tones correspond roughly to an alphabet.

http://www.chakoteya.net/StarTrek/58.htm[2]

So this establishes that Spock knows of several cultures that are Vulcan offshoots, including at least two cultures, highly different from modern Vulcan culture, that use musical notes as words.

And in those two mentions of cultures descended from Vulcans Spock doesn't express any surprise or puzzlement about how those cultures on distant planets could be descended from Vulcans.

So when Spock says:

Vulcan, like Earth, had its aggressive colonising period. Savage, even by Earth standards.

I have always assumed that Spock was referring to Vulcan's aggressive colonizing period of interstellar travel. Which makes me wonder if Earth also had an aggressive colonizing period of interstellar travel.

So I assume that Vulcan had a vast space empire stretching for tens, hundreds, or thousands of light years from Vulcan. And then some cataclysmic event like a foreign or civil war caused "The Fall of the Vulcan Empire" reducing the surviving Vulcans to a few primitive tribes on Vulcan and some of their colony planets. And it took the survivors thousands of years, or tens of thousands of years, or hundreds of thousands of years, to slowly rebuild civilization and rediscover warp drive.

Presumably Surek, who taught logic and emotional control to Vulcans, lived on Vulcan after civilization had been rebuilt, though possibly long before Vulcans rediscovered warp drive.

In "The Savage Curtain" an image of Surek speaks:

SURAK: In my time on Vulcan, we also faced these same alternatives. We'd suffered devastating wars which nearly destroyed our planet. Another was about to begin. We were torn. But out of our suffering some of us found the discipline to act. We sent emissaries to our opponents to propose peace. The first were killed, but others followed. Ultimately we achieved peace, which has lasted since then.

Clearly the Vulcans in Surek's era were advanced enough to have fission bombs at least, but he says nothing about colonies on other planets or in other star systems that might have also been threatened by the oncoming war. Thus it is possible that Vulcan had not yet reinvented warp drive in Surek's era.

Many scientists would claim that warp drive is totally impossible and thus it would take a civilization an infinite amount of time to invent warp drive after inventing fission bombs, so Vulcan in Surek's era could have already had fission bombs or worse decades or centuries or millennia before reinventing warp drive.

So here is the hypothetical chronology so far:

Vulcans evolve on the planet Vulcan, presumably tens or hundreds of thousands of years ago.

Or:

In "Return to Tomorrow" 5 February 1968:

SPOCK: This vault was constructed about a half a million years ago. About the same time the planet surface was destroyed, if our sensor readings are accurate.

SARGON: Because it is possible you are our descendants, Captain Kirk. Six thousand centuries ago, our vessels were colonising this galaxy, just as your own starships have now begun to explore that vastness. As you now leave your own seed on distant planets, so we left our seed behind us. Perhaps your own legends of an Adam and an Eve were two of our travellers.

MULHALL: Our beliefs and our studies indicate that life on our planet, Earth, evolved independently.

SPOCK: That would tend, however, to explain certain elements of Vulcan prehistory.

SARGON: In either case, I do not know. It was so long ago, and the records of our travels were lost in the cataclysm which we loosened upon ourselves.

http://www.chakoteya.net/StarTrek/51.htm[3]

So possibly Vulcan was colonized by Sargon's people about 600,000 to 500,000 years ago, and presumably lost contact with the home planet, and possibly was devastated in the war, about 500,000 years ago.

If that speculation is correct, Vulcans have lived on Vulcan for about 500,000 to 600,000 years.

And if that speculation is not correct Vulcans probably evolved on Vulcan an unknown time in the past. But it probably took them tens or hundreds of thousands of years to go from hunter gatherers to build up a space travelling civilization.

So the Vulcans discovered warp drive and colonized and conquered many planets and built up a space empire which was destroyed, probably in a foreign or civil war, thousands of years ago, or tens of thousands of years ago, or hundreds of thousands of years ago.

And over many thousands of years some of the non Vulcan cultures and some of the Vulcan colonies, and the Vulcans on Vulcan itself, slowly built up new civilizations. Both Vulcan and Romulus rediscovered warp drive and interstellar travel.

According to Star Trek: First Contact 22 November 1996 Vulcans already had interstellar travel sometime before first contact with Earth in the year 2163 in the calendar used in the TNG era.

In the "Carbon Creek" episode of Enterprise 25 September 2002 T'Pol tells a story that Vulcans including her great grandmother T'Mir, were already observing Earth when Sputnik I was launched.

T'POL [OC]: They'd gone to Earth to investigate the launch of its first artificial satellite, called Sputnik.

TUCKER: Hang on. T'Mir was your great-grandmother? I'd be the last person to question your math, but aren't you missing a few generations? Sputnik was two hundred years ago.

ARCHER: Don't forget how long Vulcans live.

http://www.chakoteya.net/Enterprise/27.htm[4]

But in DS9 "Little Green Men" 13 November 1995, Quark, Rom, and Nog find themselves crashed on Earth, at Roswell, New Mexico, and thus apparently in AD 1947.

(We pan over to a calendar with a picture of Rita Hayworth for July 1947.)

And:

NOG: No. They don't have universal translators. I recognise those uniforms from my guidebook. They're from the twentieth century.

ROM: The twentieth century? You mean we travelled back through time?

NOG: More than four hundred years. Those are military uniforms from one of the old nation states. Australia or something.

And:

QUARK: Forget this timeline. The one we're going to create will be better. Once we get things in order here, we'll contact the Ferengi homeworld and sell them our ship. The Ferengi will have warp drive technology centuries before humans or Klingons or even the Vulcans. We'll establish an economic empire beyond even Grand Nagus Zek's wildest dreams. And I'll control it all.

So Quark claims that Humans, Klingons, and even Vulcans won't have warp drive until centuries after 1947, which conflicts with Vulcans having warp drive in 1957 and 2063 in other productions.

And presumably the age of Surek was some time after Vulcans invented fission bombs at least but some time before they reinvented warp drive.

Nobody knows when the Romulans reinvented warp drive, but it should have been some time before the Romulan War.

In the TNG epsode "Gambit Part 1" 11 October 1993 the Enterprise investigates the Barradas System:

RIKER: What else do we know about Barradas Three?

DATA: The planet was used as an outpost for the Debrune approximately two thousand years ago. The Federation's Archaeological survey has catalogued numerous ruins on the surface.

http://www.chakoteya.net/NextGen/256.htm[5]

If the Debrune had interstellar travel approximately 2,000 years before 2370, or about AD 370, their civilization must have collapesed long ago, otherwise they would rule all of the Federation by the era of TOS and everyone would speak Debrune.

And:

DATA: Perhaps these artefacts have a special value to the Romulans.

WORF: The Romulans?

DATA: These structures were built by the Debrune. That race is an ancient offshoot of the Romulans. The ruins on the planet where Captain Picard was killed were also Romulan in origin.

WORF: The leader of the group that attacked us was Romulan. Perhaps they are controlling the mercenaries.

And:

DATA: There are several archaeological sites in this sector containing ruins which are Romulan in origin. These are the locations that were attacked by the mercenary vessel.

LAFORGE: Looks like they did a pretty thorough job.

DATA: The only sites not been attacked were on Calder Two, Yadalla Prime, and Draken Four.

LAFORGE: Yadalla and Draken are at the far edge of the sector, but Calder Two? That's less than a day from here at maximum warp.

Anyway, if the Romulan offshoot culture the Debrune was already capable of interstellar travel about 370, the Romulans should have been a much older culture. And thus the Romulans should have ruled all this region of space for thousands of years and everyone should be Romulan subjects long before the time of TOS.

So the approximate timeline for Romulus should be:

Thousands of years ago the Vulcans develop interstellar travel and conquer alien planets and colonize other planets with Vulcan setters.

Sometime later the Vulcan interstellar civilization collapses, probably in some vast space war.

After probably thousands of years, descendants of Vulcans on the former Vulcan colony on Romulus develop the first Romulan star travelling civilization.

Sometime later the Romulan interstellar civilization collapses, probably in some vast space war.

After probably thousands of years, descendants of Romulans on the former Romulan colony on Debrune develop a Debrune star travelling civilization. The Debrune establish an outpost on Barradas Three about 2,000 years before TNG or about AD 370.

Sometime later the Debrune interstellar civilization collapses, possibly in some vast space war.

After probably more than an thousand years, the Romulans on Romulus develop a second Romulan star travelling civilization, an unspecified number of years, decades, or centuries before the Earth-Romulan War.

And there is a somewhat similar chronology for Vulcan.

In Enterprise "The Andorian Incident" 31 October 2001, the Enterprise visits a planet with a Vulcan monestary P'Jem:

ARCHER: I take it this P'Jem is under Vulcan jurisdiction?

T'POL: It's an ancient spiritual retreat. A remote sanctuary for Kolinahr and peaceful meditation.

Later:

TUCKER: You say this is a place to purge emotions? Looks like somebody had to purge pretty bad. He bashed the door in.

T'POL: The temple is almost three thousand years old, Commander. You can't expect it to be in pristine condition.

http://www.chakoteya.net/Enterprise/07.htm[6]

So if the P'Jem sanctuary was almost 3,000 years old in 2151 it should have been built about H.E. 9251 to 9751, or 750 to 250 BC.

In the episode "The Forge" 19 November 2004 Vulcan Ambassador Soval tells Admiral Forrest:

SOVAL: We had our wars, Admiral, just as humans did. Our planet was devastated, our civilisation nearly destroyed. Logic saved us. But it took almost fifteen hundred years for us to rebuild our world and travel to the stars. You humans did the same in less than a century. There are those on the High Command who wonder what humans would achieve in the century to come, and they don't like the answer.

http://www.chakoteya.net/Enterprise/83.htm[7]

If it took about 1,450 to 1,500 years for Vulcans to rebuild their world and travel to the stars after the conflicts in the age of Surek before the Great Awakening, the Great Awakening should be 1,450 to 1,500 years before Vulcans began to travel to the stars.

If AD 1947 was centuries before Vulcans had warp drive, as Quark claimed, and centuries means at least 200 years, the earliest possible date for the Great Awakening should be about AD 647 to 697.

If Vulcans were already observing earth by 2063 as in Star Trek: First Contact, the latest possible date for the Great Awakening would be about 563 to 613.

If Vulcans were already observing earth by 1957 as in Enterprise "Carbon Creek", the latest possible date for the Great Awakening would be about 457-507.

Later:

T'POL: This is a desert called the Forge. The route's the one Surak supposedly followed.

ARCHER: Eighteen hundred years ago?

And in "The Awakening" 26 November 2004:

SURAK: You know who I am, Captain. So much death. Hard to believe this will become known as the Time of Awakening.

ARCHER: That was eighteen hundred years ago. This doesn't feel like a dream.

http://www.chakoteya.net/Enterprise/84.htm[8]

So if "Eighteen hundred years" before AD 2154 is 1,750 to 1,850 years, Surek walked the Forge shortly before the Great Awakening about AD 304 to 404. if "Eighteen hundred years" before AD 2154 is 1,700 to 1,900 years, Surek walked the Forge shortly before the Great Awakening about AD 254 to 454. That is assuming, of course, that the 2154 date of the fourth season of Enterprise is AD 2154.

And Vulcan would have begun travelling the stars about AD 1704 to 1954.

If Vulcans traveled thorough interstellar space to build the P'Jem Sanctuary about 750-250 BC, and Vulcan was devasted about AD 300-400 and didn't resume interstellar travel fo ralmost 1,500 years, Vulcan should have stopped interstellar travel some time in the centures after P'Jem was built.

The image of Surek didn't mention anything about space colonies in "The Savage Curtain", and if Vulcan had centuries old interstellar colonies at the time of the grat wars on Vulcan in the era of Surek, then the Vulcan interstellar colonies would have eventually built starships and traveled to the devastated planet Vulcan and helped rebuild it.

If Vulcan civilzation had interstellar travel continuously from 750-250 BC to the present the Vulcans would have become the dominant interstellar power in this part of the galaxy and everyone would be ruled by Vulcan.

So the combined history of Vulcan and Romulus would go like this:

Thousands of years ago the Vulcans develop interstellar travel and conquer alien planets and colonize other planets with Vulcan setters.

Sometime later the Vulcan interstellar civilization collapses, probably in some vast space war.

After probably thousands of years since the fall of the first Vulcan interstellar civilization, descendants of Vulcans on the former Vulcan colony on Romulus develop the first Romulan star travelling civilization.

Sometime later the Romulan interstellar civilization collapses, probably in some vast space war.

Sometime later, Vulcan redevelops a high civilization and begins a second period of interstellar travel. The sanctuary at P'jem is built about 750 to 250 BC.

Sometime later, the second period of Vulcan interstellar travel ends for some reason. P'Jem and possibly other Vulcan colonies might be abandoned or might continue.

After probably thousands of years since the first Romulan interstellar civilization, descendants of Romulans on the former Romulan colony on Debrune develop a Debrune star travelling civilization. The Debrune establish an outpost on Barradas Three about 2,000 years before TNG or about AD 370.

Sometime later the Debrune interstellar civilization collapses, possibly in some vast space war.

Meanwhile, on Vulcan, Vulcans have an advanced enough civilization to build fission bombs or worse, though they are probably not yet advanced enough for interstellar travel. A series of wars devastates Vulcan. About AD 300 to 400 Surek walks the Forge. Surek begins the Great Awakening.

Vulcans begin the slow process of rebuilding.

After probably more than an thousand years, the Romulans on Romulus develop a second Romulan star travelling civilization, an unspecified number of years, decades, or centuries before the Earth-Romulan War.

Vulcans develop interstellar flight about AD 1750 to 1900.

This may seem like a very complicated history, but don't blame me, I don't write Star Trek episodes and movies, I only try to make sense of them by interpreting them in the most logical way I can.