US Dept of Education says 3-5 math problems are more than enough to show if you got it or not (now of course 3 calculus problems are completely different than three addition problems so I even go as far as to say 2 hard problems up to 5 practice in straight calculation problems) is enough. And if you give elementary school kids a few quiet minutes at the end of the day for "work time" you might not need to bring it home (depending on the math task).
If I ruled the world...2-5 math problems, maybe a challenge problem for those who actually like that kind of stuff (optional) and reading would be the only allowable homework (reading could be reading class assignments, own book, something for a content class)...with the occasional review/study for test(with help on learning HOW to study for a test). Everything else (including projects and papers and such) should be done at school.
