Skyrim - over 2500 hours, sadly.
Fallout 3 - also around 1500 hours play time.
Oblivion - Did a lot of modding for this, mainly cheats though.
The Sims 3 - Seriously, I find designing buildings therapeutic.
Portal I and II - Love the puzzle solving aspects.
Bioshock 1, 2 and Infinite - Still to...