There is surprisingly little good research on the educational value of homework, but what there is has resulted in the consensus (routinely ignored by educational institutions) of:
1. 10 minutes of homework per year of grade.
2. Homework in elementary is for the purpose of developing soft skills. Given good classroom instruction, it provides negligible added value academically, and should not affect grades other than those of the "citizenship" category.
3. Homework has some academic value at the secondary level, mainly for lengthy readings in literature and history, and somewhat for readings in science. Some data suggests not only diminishing returns for greater homework time, but negative returns after a certain point. Middle school students should not be exceeding 60-90 minutes per night. High school students should not be exceeding 2ish hours a night. More than that is correlated with reduced academic outcomes. (Granted, this may be because the population which puts in more than 2 hours a night is enriched for students with learning difficulties, who tend to achieve at lower levels.) Homework should affect grades only in a positive direction, or not at all.
Synopsis of frequently-cited meta-analysis on homework and achievement data, by the primary author of the research:
http://www.sedl.org/pubs/sedl-letter/v20n02/homework.html
