Your talking about looking into the future (You will need time machine to do that) Tech changes very fast, game engines are being developed, anybody that will give you a number of years are wrong. If you have a 1440P monitor buy SLI 970's and see what the future brings. Look at just the last year 780ti 500 bucks and now a 970 with very close performance for 350.
If you have a 1080p monitor now buy a single 970 when you upgrade the monitor add another 970.
It's a never ending cycle.