Well if you run at 4k or on a 144Hz - 1440p Monitor, even actual titles can't be run on a single GTX980Ti. Witcher 3 for example sits at about 65fps with everthing maxed out on a 1440p Monitor. That may be good fluent gameplay, but if you buy a 144Hz Monitor, you would at least want some graphics card that can make passable framerates for it.
On the other side, if you game on a 1080p Monitor with 60Hz and you want to reach 60fps it should probably last about 3-4 years. You can't say that for certain, because the next big invention could be already around the corner. If we consider recent development of games, then 3 years should be possible, but if by some kind of miracle the GPU performance increases by 400% within the next chip generation and games make use of that then it's not even one year.
The third category could be the console type quality of 720p up to 1080p depending on the game with 30 to 60fps and reduced details. If that's your goal then the card should last for about 5 to 10 years. Even a 5 year old gtx480 can destroy a ps4 right now.