Honestly, we can never tell. Games will be continuously developed, some may be GPU-intensive while some may be not. Currently, with that 30-60 FPS, your GTX 980 Ti can perform up to 1440p and, to some extent, 4k. It'll max out all games at 1080p (and would seem much as an overkill). In fact, a GTX 970 might just suffice. So, with a GTX 980 Ti at 1080p? It'll probably go a loooong way at that resolution. Exact number of years? Nobody can tell.