On average, I'd say a game usually takes me 15 hours or so. But I'm constantly at war. Plus, if I'm losing a war, I tend to re-evaluate my strategy and go back to a previous save (I save a new file every 10 or so turns) and see if I can do things better. That's just on a standard size map in Civ4. I don't know how long my games last with larger maps yet because I have yet to try them.
Although my games in Civ2 and Civ3 used to last much, much longer, 50+ hours as I played on huge maps all the time.
And I have never won a diplomatic/cultural/space race in any Civ game I've played, IMO conquest is the only way to go. I can't help the temptation of actually using my units rather than leaving them to atrophy in my cities.
