Just my opinion: I'm 26 yrs old and when I was in school I was always in honors classes from 5th grade on.  When we were first put into honors classes every year, there was always a meeting with the honor students and parents explaining how much homework time would be expected every night as well as the other expectations for the classes.  It was at that time the student or parent could opt out of being in the honors program if they did not feel like they could handle the pace and amount of work accompanied by being in the advanced classes.  I average 2 to 3 hours of homework every night from 5th grade on (high school it was even more - but I went to a college prep high school)  .  Eventually you became accustomed to the work and not all times of the year were that bad.  The first half the year always seemed the worse because you are just getting back to school learning a whole bunch of stuff so fast.  But like I said I got used to it.  One advantage I had too was that I enjoyed school and school work (probably was considered a dork! 

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My suggestion would be that if your son is in the honors program, that it should be expected that there is more homework and that if he becomes stressed with the amount of work perhaps ask for him to be put in regular classes for year or two till he's ready to be in the advanced. Also I truly believe that it has to be a choice the child makes not one that is forced upon the child whether it be by the teacher, principal, parent, etc.   By 5th grade, children know what they can handle or at least I did.   
Also one other opinion, so please don't shoot, I am all about students getting help with their school work, but I think it should be done by the teachers or the school.  It should be done by people who truly know the cirriculum.  The reason I say this is because from my first hand experience.  My dad is a high school teacher and growing up I would ask him from help with my homework.    However, since my dad did not know what my teacher was actully teaching us in class or what the specific directions were for doing the homework...it was always hard for him to truly help.  Just reading the text your child brings home will not show you what he is learning in class.  Sure it might give you an idea but it does not include the teaching that was done in the classroom.  And unless your child takes excellent notes (which I don't know any 5th graders including myself at the time who did that) you cannot know what was actually said, taught, etc.   Either way, I learned that doing homework by myself got me better grades than when my dad helped me.    
Also I tutored my cousin throught out her schooling years. (She just graduated from HS in June).  However, she was one of the children out there that school never came easy for her due to developmental issues.   And I found that she used me as her scapegoat.  I was the one that helped her but really I just ended having to do the homework for her because she could never get it.  This is the worse thing that can be done for a child...when people do the work for the child.  A lot times, I found it easier to just do it for her because it meant we would get it done sooner.  I had told her and her parents about this and that she needed professional help but it was no use.  Because I could never explain it to her properly, that the teacher should be the one helping her, but to them school was something she needed to get through.  School was never something she would be good at and never would be.  While I think there are plenty of people out there in the world where school just "isn't there thing", people should still get the help they need from the right people.  Just becuase I have a college degree does not mean I have the means or the knowledge to help youngsters with their school work.  Times change and the things kids learn in school and the way the are taught to learn is different from what we were taught....(i.e. new math - some children do not even learn how to do long division anymore) .  We can't expect that we can know how to help a child with their homework.   
Just my opinion....so this was so long...just something I feel strongly about.