I think I heard somewhere it was 12-15 hours depending on your play style. (though it could be longer if you take your time and search ever nook and cranny). If I find where I saw that I'll come back and post the link, so take it with a grain of salt until further notice. But I'm 90% sure thats what I heard from a site
This game has been in development 4 years with THQ working on it for 3, then Obsidian for the last year. This has me worried because THQ has made some crappy games and Obsidian has taken over but at the same time, this game was supposed to be released a while ago, so THQ is the main developer. Matt and Trey boil down to 14 episodes a season (year) except for this last season (10). This is about 4 to 5 hours a year after edited content. Knowing Matt and Trey and being a die hard South Park fan, I am going to say between 12 and 15 hours of main quests with 10 hours of side quests. I will be disappointed with anything less than 10 hours of story and 5 hours side.