As a casual, there are no minimum hours you have to be given every week. Your hours will depend on how many hours the employer needs worked. You could work 15 hours one week and none the next week.
However, if called in for a shift, the minimum you must be paid is three hours if you work in a retail store and two hours if you work in a fast food outlet.
Click here for ACTU Work Site for Schools Fact Sheet which provides more information about casual and part-time work.