I'm in the habit of playing the same games for years, especially the online multi-player games where building characters takes time. So 4-6 years is not unheard of for me. Since the game hasn't changed, the fps hasn't changed, the gpu doesn't need changing. I'll also be gaming on 1080p until they are as obsolete as CRT monitors, so upgrades to giant gpus are also unnecessary.
So when do I change gpus? When they die, when they can't handle the game I want to play or when 4k becomes the 'new' norm.