Research mentors are typically flexible when it comes to scheduling hours in the lab.  They understand that you are a student first.  However, a frank discussion about your availability with a potential mentor is important before any commitment is made.  It is generally recommended that you have 6-10 hours available per week for research activity.  For some projects, students are able to put in hours on the weekends or in the evening, and some have more restricted weekday hours available.  Be sure to ask.  It is typically best to plan a weekly schedule ahead of time and stick to it — just like a class!  Students have the best experiences when they are consistently committed.