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.
I am not counting the other things a parent does to enhance a child's learning experience as homework time - so we all might be saying the same thing.
I will give a small example which might illustrate having an enhanced learning experience. One of my volunteer assignments was to teach children math using coins. My son and daughter were naturally motivated to understand how much money they had because I had them save money buy things at a very young age. I was shocked that some classmates did not understand denominations and others could not do the math. My husband made coin dice for the classroom kids. They rolled the dice and added the values to figure out how much money they had on the coins. This was a big hit with the kids.
What I didn't want to do was do homework for my kids or give answers. Discovery is how children learn. My kids ended up proud of what they could do on their own. When they failed - we discussed taking ownership of failure.
It is amazing how even two kids from the same parents can vary greatly in personality and how they learn. It takes time and energy to find the best way to nurture each child.
Oh yeah, and it feels good too. Remember that time when you spent hours explaining something to your kid and he suddenly got that "Eureka!" look? It's hard to quit that.