For a nine year old (3rd or 4th grader), I'd say no more than 30-45 minutes a day. For middle school, 60-90 minutes. For high school, 2 hours, depending on the course load. Sometimes middle school and high school homework goes in fits and spurts if there are projects and papers due. So there might be more homework some days, less on others. Also, I know I spent at least an hour each night on advanced math homework (trig and calculus) in high school.
Also, as a teacher, I strongly believe that homework should be practice work that the student can complete INDEPENDENTLY, with minimal parent help, not new material.