6-9 hours is good for me

I would take 0 minutes of single player if the game offered a great multiplayer! Single player for everything but RPG's is no where near as important as MP IMO.

I want as much bang for my buck..... I always feel gypped when a game I've been anticipating can be completed in 10-15 hours (essentially a weekend)....yes, I'm looking at you NSMBWii!

12 hours is a bear minimum (including a second playthough if it's good enough). I prefer my games to be around the 15-20 hr mark, 40hrs minimum for a pure RPG.

If a game has good replay value (more than 3 playthroughs recommended) than 4-5 hours is also fine like the episodes of Half-Life 2. There is always stuff you can miss out first time round.

I would say infinity.