I also have a third grader in FCPC. Here's what Fairfax says:
http://www.fcps.edu/about/homework.htm (I cut/pasted the applicable part below):
On average, daily homework across disciplines should not exceed:
Primary (Grades 1-2) = .5 hour
Upper Elementary (Grades 3-6) = 1 hour
Middle School Level (Grades 7-8) = 1.5 hours (30 minutes per core subject)
High School (Grades 9 - 12) = 2 hours *
* The amount of homework may be greater for students enrolled in GT, AP or IB classes.
That having been said, I believe our teacher at open house told us that if the kids are working for I think it was 40 minutes (really focused, undistracted working) and still don't have their work done, it's ok to stop. She said if they don't finish an assignment because of that, to write her a note explaining the situation. She said that homework is only meant to reinforce what is done in class and should not take too long.
I honestly don't really know how much time my daughter spends on homework. She spends part of her time in SACC doing homework and then frequenly finishes her work at home before dinner. She is also supposed to read for 20 minutes every day, which she does before bedtime. The homework she had received so far has mostly been spelling assignments (write words several times, use words in sentences, etc.) and a math set. That's about it.