It varies for every person - as some people have no RA requirements, others have 20 hours a week - some people TA, some don't, etc. I have really varied this semester, but for a while I tried to treat it like a normal job: work every day from about 8am to 6pm, and then do nothing grad-school related on nights or weekends. This includes going to class, classwork, RA hours, research, etc..