Based on history, and if you stay at 1080p, it should last you several years, especially with 6GB VRAM as more games are allocating more VRAM for use even at 1080p. We can see that today just in GTA5 and Skyrim with mods using beyond 4GB. But nobody has a crystal ball. All we can do is look into the past.
To put that into perspective, my 4-year old GTX 680 SLI 1080p rig would still be a decent build for today's games except for the fact it is limited to 2GB VRAM. Most games I play allocate between 2-3GB VRAM for use at 1440p with ultra quality settings and some form of AA enabled...Witcher 3, BF4, Crysis 3, Far Cry 4, Battlefront, Project Cars, Assetto Corsa, DiRT Rally).
The 970/980 replacements will be out later this summer and based on history, the GTX 1080 or whatever it will be called will be equal to or even surpass a 980Ti for less money. If you need a card right now, you can't go wrong with a 980Ti for several years to come at 1080p. The flagship 980Ti replacement won't be out until next year some time.