The rule in our school is 10 minutes per grade. So, a 4th grade student should get 40 minutes homework. An average student should be able to complete that in that time, and an adult could do it in probably 15 minutes.
Is this more work than you can complete in about 15-20 minutes? If it is, he's getting too much work, imho. He's been in school all day and still has so much to complete.
Usually when a child is having trouble with completing homework, it means he/she does not comprehend what is to be done (or the child is unable to complete the work easily). I'm assuming most of the time spent on "homework" is not actually used in completing homework but in wasting time. Is this correct? If so, then the issue is not the amount of homework, but that your son is having difficulty with the comprehension and/or completion of the assignments.
By the way, many teachers do not assign additional homework - most "homework" is classroom assignments that the child did not complete (for whatever reason). Eight years of age usually means Grade 3 and this is often the age when learning issues become more noticeable. I really think you need to have a "talk" with your son's teacher. Hopefully, she/he will be able to shed more light on this situation.
Both rockrose and jdtm give possible answers. Let me give a third possibility (and there could be more). I post a lot on the ADHD forum. This certainly has the symptoms of someone with ADD. A person with ADD can easily be missed by the teacher - especially if the child has decent intelligence and is a nice kid. Its the hyper ones that always get noticed by the teacher. Somewhere around 4th grade, the child finally hits the point where innate intelligence will no longer help with math problems like fractions, and grades began to sufffer. Essentially if the child can't focus at home (assuming he is working in a quiet place, no distractions, etc.), he will have a hard time focusing at school and miss out on the lessons. This will lead to lower grades, and if the kids cares - a great deal of frustration.
The reason I mention all of this is that there are different things that can be done depending on what the problem is. Its finding out or figuring out what the problem is - is what is important. Do so for ADD is kind of tough. Most web sites focus on ADHD. On the ADHD forum, one book I recommend a lot is- "The ADD/ ADHD Answer book," by Susan Ashley. On page 3 she lists the common symptoms. I have used a slightly different list from this web site as I didn't want to type it all out. That site is - http://www.healing-arts.org/children/ADHD/#ADD - Its a good site, just kind of hard to wade through. Anyway, the symptoms are:
1. often fails to give close attention to details or makes careless mistakes in
3. other activities.
2. often has difficulty sustaining attention in tasks or play activities.
3. often does not seem to listen when spoken to directly.
4. often does not follow through on instructions and fails to
1. finish schoolwork
3. duties in the workplace (not due to oppositional behavior)
4. failure to understand instructions
5. often has difficulty organizing tasks and activities
6. often avoids, dislikes, or is reluctant to engage in tasks that require sustained mental effort (such as schoolwork or homework)
7. often loses things necessary for tasks or activities (e.g., toys, school assignments, pencils, books, or tools)
8. is often easily distracted by extraneous stimuli
9. is often forgetful in daily activities
If things things sound familar, then its probably worth doing some more research by checking out the sources mentioned above.
Hope this helps. I am sure all of us will be glad to answer any more questions by you. Best wishes.