Our school policy is 10 minutes per grade level. Example: 1st grade=10 minutes, 5th grade=50 minutes. By the time they get into middle school, 8th grade let's say, it could mean 80 or 90 minutes.
In my experience (as a teacher and a parent), this is a good homework measure. It is generally true for my children, BUT you have to take into consideration how your child handles homework. My son uses all his time during the school day to get as much HW done as possible. My daughter will dilly-dally during her free-time/structured work time and come home with an hour of homework. Once she begins pouting and complaining, the one hour worth of homework turns into two or more. My youngest son has difficulty focusing in general (he's autistic), so he's a completely different egg altogether.
Hope this helps!