Woa... I guess some people don't know the expression "if you don't have anything nice to say, don't say it at all." So, let me try to give you some supportive advice, not criticism. (And I just can't resist... Heidi's post has about 8 errors in it... apparently she didn't spend enough tie doing her HW when younger!)
I am a 4th grade teacher. Our school has homework guidelines by grade level. I am supposed to give, on average, 45-50 minutes. My kids can also earn 10 minutes of choice time at the end of the day, during which time they can work on homework if they so choose. 5th grade is supposed to give 55-60 minutes. Of course, every child works at a different pace, so this is just an estimate, but anything significantly more than this sounds like too much to me. Sometimes certain kids might have slightly more than other kids if they chose to fool around during class and not finish their classwork. You should make sure this is not your son's case. If not, you should talk to his teacher.
There are many recent studies that put homework into question, and, in fact, refute it. Homework can really make kids dread and dislike school. I don't think we're at the point in our society to abandon homework- we may never be- but, in my opinion, teachers should be careful that homework is not causing an undue amount of stress and time. And to say that a 5th grader should just "suck it up" and deal with it b/c he'll have to deal with it in HS and college is ridiculous. 10 year olds are not in college- they are in elementary school. What's next, should we start SAT prep in kindergarten?
I have only received one complaint about the quantity of homework I give, and it isn't so much about my assignments as it is about the work habits of the girl, who wastes time in class and puts up a fight at home, so it is often difficult to get it all done. Therefore, I am in frequent communication with her mom and we have set up a plan. If she spends 20 uninterrupted minutes on one assignment, she should take a short break then move on the the next, even if it is not all done. I am perfectly satisfied with that arrangement. Any time a parent writes a note attesting to the effort put in, I'm ok with it. I used to give projects to do over vacations, which I believe were justified and I did not hear complaints from parents, but I have even scaled back on those in the last couple of years.
When in doubt, I would suggest talking to the teacher. Explain your nightly routine. He/she might not realize how long it is taking kids to complete it. read the school handbook, if one exists, and see if there are established timeframes.