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
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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.