The two major trends in GPUs have been more horsepower for the same amount of money and marginally less power use. A GTX 770 is roughly equal to a 680, a 670 was roughly equal to a 580 (if i remember correctly), a 570 was roughly equal to a 480 (again, iirc). Cost per X amount of horsepower goes down.
I personally upgrade every other generation (260, 570, 770) and have had plenty of graphics power. Granted, your results may vary. I think a GTX 770 will last you three years. If you upgrade to 4k, consider a newer graphics card.
That said, you don't always need more horsepower. Bear in mind most gamers have less than a GTX770 in their machines. High end graphics cards are still somewhat of a niche market. I have a buddy who runs Borderlands 2 on two 9800gt's! Talk about a blast from the past.
I don't think graphics are going to change substantially in the next three years. At 1080p, not enough is going to change in three years that you have to move from maxed settings to low settings. A GTX 770 won't suddenly become a stuttering laggy mess in three years at 1080p.
Hell, my cpu came out four years ago and it still runs just fine. First gen i7. Perhaps a little lacking in single threaded performance, but it still runs great. I'd expect the same to apply to current generation graphics cards.