It's far enough in the future that he can travel through time, but
not too far because Rusty's still alive to kill Cable's family
His future is most likely set in 2068.
"50 years from now, you're very dead. Your entire generation fucked this planet into a coma."
Deadpool 2 takes place in 2018, 50 years from that is 2068.
On this website here: http://marvel.wikia.com/wiki/Nathan_Summers_(Earth-616)
It says that he is sent to the late 37th or early 39th century, because he was infected with the Techno-Organic Virus.
But on this other website: https://comicvine.gamespot.com/cable/4005-2157/
It just says that he was sent 2000 years into the future to survive the Virus.
His Wiki Article says that he was born in the Year 1986. https://en.wikipedia.org/wiki/Cable_(comics)
By doing some simple Math, my guess would be that he spent his early life in the 40th Century or the Year 3986.
Please note that i am no expert in Marvel Lore.