That is a lot for first grade, IMO. I gave 1-2 assignments a night, depending on the amount of time it would take. Homework for a first grader should only take about 10-20 minutes a night. That is all their attention span can handle. I have read and heard from numerous sources that it should be about 10 minutes per grade level, but then you need to build in some procrastination time and some time for a student who just might be a little slower at completing the work. 90 minutes of homework is what high schoolers should have!
Here is a link from a reputable source: http://www.nea.org/tools/16938.htm