DS7 is in 7th and we have a rule, 2 hours max. I told his teachers that because of his age I wouldn't ask for anything more than that and they agreed. If his homework doesn't get done, they all know that it wasn't because he didn't try and they accept that. He just can't write as fast as your average 7th grader so it takes him longer to get it done.
The rule at the school is that middle school gets 2 hours, high school gets 3, but in all fairness most of the high school kids are dual enrolled at the college so their homework isn't really determined by the school.