A 2012 study found no relationship between the amount of time spent on homework and grades, but did find a positive link between homework and performance on standardized tests. A 2006 analysis of homework studies found a link between time spent on homework and achievement, but found it was much stronger in secondary school versus elementary school. That analysis also found that for junior high school students, homework reaches the point of diminishing return at around 90 minutes, and between 90 minutes and 2.5 hours for high schoolers.
via Homework debate: Too much, too little or busy work? – CNN.com.