I'm thinking its more like 20 hours of gameplay. Definitely near the 10-12 hour mark now and have at least awhile longer to go. I know I saw a zombie chef in one of the commercials and haven't come across that in the game yet.
I have done most of the side missions that popped up along the way so that might be why I'm getting a longer first play through. Its definitely worth it for any south park fan. So many characters, so many jokes.