Ideally, parents shouldn't spend any time on their children's homework. I can understand answering occasional questions and giving encouragement but nothing more. Children learn by figuring out homework on their own. Otherwise, why have homework?
The problem here is that the parents best able to help their children are more likely to have kids in the top half (who don't need help) and the ones least able to help have the kids who need help the most. This can be solved with a restructuring of our school system but very few schools are willing to try. Instead we stick with the ridiculous age == learning level method.