Everyone has a different expectation for how many hours they would like to work, and the interviewer is simply learning what your expectations are. Before arriving at your interview, have a number of hours in mind that will work well with your schedule. If you need to work 35 hours a week to pay your bills, share that you expect 35 hours per week. If you are a student and want to work 10-15 hours per week around your school schedule, share this. Whatever your personal needs are, simply share them with the interviewer.