It all depends on the genre. For anything in the vein of action-adventure, beat 'em up, FPS, or platformer, 8-10 hours is about what I like. Anything more and, unless the game is something special, I start to get bored.
For Zelda-likes, I can do more along the lines of 20. For RPGs, maybe 30, or up to 40 if the game is really good.
For horror games, my threshold is lower - more like 6 or 7. You don't want horror games to be long, because they can quickly cease to be scary if they're too drawn out.
As for gaming sessions, I prefer 30 minutes, at the most. I like being able to drop a game and still save my progress in a short time if I have something going on with friends or classes.
Remember that quantity does not equal quality, and that many longer games are simply padded with filler. I'd rather play a tight, balls-to-the-wall 8 hour game than a dull, scattered 20-hour game.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
