I don't know what the actual average is but as a parent of middle-schoolers that sounds right. I've always fought my kids' teachers to keep them from wasting too much time doing homework but if I didn't they'd probably have at least this much. A friend of my wife had to email her son's 6th grade teacher the other day explaining that the son was sent to bed early and could "only" spend 4 hours on the homework that _day_.
But many parents report that their kids require far longer to complete the assignments. When my son had more than 15 - 20 minutes of homework in 1st grade (5 repetitions of "write a sentence in Spanish that includes the following word") last year we told him to stop and we sent a note to the teacher. I have a friend whose kid takes 2 - 3 times longer than his classmates to finish his homework. It's hard to tell if it is helpful or not for him, but he's definitely not happy about it and he's still a small kid.
