What a great question!  Parents and teachers want to see students doing an optimal amount of work--enough that they progress without being overburdened.  Of course, the factors in this equation vary for each individual student.
 
According to guidelines from Harris Cooper of Duke University, teachers should use the "10-minute rule":
 
"'The "10-minute rule," Cooper said, is a commonly accepted practice in which teachers add 10 minutes of homework as students progress one grade. In other words, a fourth-grader would be assigned 40 minutes of homework a night, while a high school senior would be assigned about two hours. For upper high school students, after about two hours' worth, more homework was not associated with higher achievement.'"
 
So, a third grader should have roughly 30 minutes of homework.
 
As a former classroom teacher, I would agree but with the caveat that this does not include study time.  Some skills, such as foreign languages should be practiced for 10-15 minutes every night.  And some time must also be set aside for steps in long-term projects, such as essays or science fair projects.
 
Additionally, I highly recommend that each child read a book of his or her choice for at least 20-30 minutes a night--though if you can find the right book, hopefully your child wants to continue reading more!
 
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Candace Lindemann, Ed.M. - JustAsk Expert
Educational Consultant and Writer
http://CandaceLindemann.com