My guess would be
settings in most games
1080 @ high/ultra 2 years
1080 @ medium/high 2-3 years
1080 @ low / medium 3-4 years
Depending on the new engines that come out you should be able to play most games at 720 @ low/medium settings for the next 6+ years but you will want to upgrade way before then. Hardware gens are lasting longer than they was 5 years ago due to them being so powerful now and using such little power. I think you'll be able to last the life cycle of the ps4 if you're willing to drop the settings over the years.
You'll be able to play most unreal engine games for a very long time. I saw someone playing the new splitercell game on a 8800M and it was running amazingly well I couldn't believe it looked so good. he was running at 720 at high but that still impressed me.
