3 years is a pretty good minimum. Considering that you really only need a 970 or equivalent for maxed-out 1080p (http://www.tomshardware.com/reviews/best-gpus,4380.html), & the 970 is usually able to get decent FPS even at 1440p (http://www.techspot.com/review/1174-nvidia-geforce-gtx-1080/), I would say that a single 980Ti is going to last you a long time, let alone SLI'd.