Check out a thought-provoking editorial about homework at TIME Magazine:
"Most of that increase reflects bigger loads for little kids. An academic study found that whereas students ages 6 to 8 did an average of 52 min. of homework a week in 1981, they were toiling 128 min. weekly by 1997...
Too much homework brings diminishing returns. Cooper's analysis of dozens of studies found that kids who do some homework in middle and high school score somewhat better on standardized tests, but doing more than 60 to 90 min. a night in middle school and more than 2 hr. in high school is associated with, gulp, lower scores."
TIME Magazine - The Myth About Homework