The professors of course want you to commit as much time as possible to the project. They don't want to waste the time training you all the techniques you need and then you are gone 2 months after the project barely started. They also want you to be availiable so they don't have to arrange time to work with you. However, realistically, you probably cannot spend more than 12 hour/week during the school year. Else, it will become a serious drag on your academic and social life.