How many hours can i expect to play with my friends?

I hear each campaign is about 2 -3 hours, and there are something like 4 campaigns. 

From what I've heard the scenarios are two hours (give or take, of course), but the replayability is where the game is going to excel.  The better you get at the game, the harder the game gets, since it automatically adjusts to your progress.

The "campaign" is infinitely long because every single time you play things will be different.