There's no requirement, really. It's all about what you want to put into it. I mean, I don't think you'd be much help going in a few hours a week- they get you working on a project, you're not there to finish it, somebody else starts working on it and you get too many chefs in the kitchen and stuff doesn't get done right. Still, to do it right would take at least 10-15 hours a week minimum, probably more if you don't already have useful skills. If you have to learn everything from the ground up (pretty much my situation) you're going to be there a lot.

Just my take on the situation- people actually participating would know better.