Everyone has a different expectation of how many hours they would like to work, and the interviewer is merely learning what your needs and expectations are. Before arriving at your interview, have some hours in mind that will work well with your schedule. Keep your answer to the point as there is no need to go into great detail about your personal life.
"At this point, I am available to work part-time, to work with my University schedule. I can work around 20 hours per week. Once the summer hits, I am available full time."