One size doesn't fit all!
A homework assignment that takes one child five minutes to finish will take another child much longer. Use the 10-minute rule described above.
If your child's homework routinely doesn't allow for completion in the amount of time that's appropriate for his grade level, make an appointment with the teacher and discuss the problem.
Teachers will often individualize assignments to meet the needs of the learner if they know about the problem. Most teachers would say that it's better to leave an assignment unfinished than to stay up past bedtime to complete it.
Remember, not all learners are created equal. It's okay to ask the teacher to modify homework assignments for your child if he routinely struggles.
Finally, don't fight the homework battle. Most teachers have a homework policy that includes consequences for missed assignments. If you've tried everything and your child still won't do the work, send the teacher an email explaining your struggles and let your child accept the consequences at school for his decision.