I'm gonna have to gago against the grain here. As someone who has trained boxing for years, no, it's not even close. It is extremely hard to do in real life, and people don't stay down more than a few seconds. In my entire boxing "career", I've taken thousands of blows, and I've never lost consciousness. I've hit other people thousands of times, and only like twice have people fallen down, and they've gotten up almost immediately.
What you see in the ring is pretty indicative. On average, it takes something like 200 punches in the head, from the best professional punchers in the world, to bring down an opponent. Those 1 hit knockouts you see in YouTube videos and highlight reels are extremely anomalous, they happen once in a million punches. Or more often, they happen to drunk teenagers, already staggering, who are fucking with professional (if retired) boxers like in the videos linked in some of the answers here.
And if you do actually hit someone so hard that you knock them out for more than a few minutes, they are almost guaranteed to have a concussion and need to see a doctor immediately. You don't get to knock someone out for 2 hours just to incapacitate them. Head trauma isn't that convenient in the real world.