First off my philosophy on teaching kids:
Optimally...each day: "I want a child to know what excellence is, what it feels like, and do something they are excellent in."
Second- Homework Philosophy:
Homework gives a child introspection, practice, and self remediation of what was covered in class. It reinforces learning...which will heighten achievement. If they are doing excellently in a subject, homework lends another opportunity to do that "extra" and show it off to their teacher, friends, parent, etc.
Homework is the real world equivalent of "going to work to finish the job". We want to teach our children to do the extra for excellence in whatever they do.
Third - how much?
Homework has to be monitored collectively by their subject/grade level instructors. For example; if in middle school "Johnny" has 6 subjects, those teachers need to articulate and come to a consensus of what is a reasonable amount...such that the net "homework time" is not an unreasonable amount. Elementary instructors can easily monitor their amount based on the needs of their students and subject areas (ex. challenging/new Math concept...give them more Math practice)
General rules of homework time:
Elementary - 10 min. per subject; more in their challenge subject for practice
Middle - no more than 30 min. per subject, Remedial: Math, Science, Reading - another 30 min. in their weak subject.
High School - depends on track and needs:
College - expect some subjects will give an hour (AP & Honors), most of us agreed on 1/2 -45 min. per subject
Vocational - an hour or more since this is project orientated
Remedial - Math, Science, Reading - up to an hour in their weak subject