Depending on the game I'm playing, anywhere from 1 to 5+ hours. RTS games, particuluarly ones that have slow campaigns (Grey Goo leaps to mind) will err toward the 5+ hour mark. I have a habit of being gunshy and wanting to build up massive armies before even exploring the map.
On the other hand, certain games that promote very fast, repeated attempts - Luftrausers, Blitz Breaker, maybe Audiosurf - will only keep my interest for 5-10 minutes at a time, though I can play, die and replay at least a dozen times in each short session.
Actiony games that require I actively engage in combat and such, that's gonna be closer to maybe 2-3 hours. Open-ended action games (Bethesda games, games like Shadows of Mordor) will only usually be 2 hours at a time, it's harder nowadays to focus on the plethora of objectives and side-quests thrown at me.