As little as possible. There is a difference between guiding them and helping them. Children should do it on their own, it's the only way a teacher knows what the child is capable of. That said, teachers should also give homework in an amount that is appropriate for independence. The only thing that helping your child with their homework solves is the parents do the work, this enabling the child.
If your child is having a difficult time with. Concept it is appropriate to reteach it or guide them to the correct solution but parents should never, ever be doing homework for their child.