The guideline I've always heard is that kids' nightly homework should be 10 minutes/grade. This would mean a 1st grader should have 10 minutes of nightly homework. Occasionally, there might be a project requiring extra time, but even those shouldn't be too demanding for first grade.
http://www.nea.org/tools/16938.htm
I agree with the previous posters to only do what makes sense to you. (I think teachers often include artsy assignments and "fun activities" trying to make homework more interesting for the kids, when really it's just an unnecessary hassle that doesn't actually help significantly with learning subject content.)
It might end up being more than 10 minutes, but it shouldn't be hours. The teacher might mark his grade down a bit, but at this age that shouldn't be a problem. Just make sure he learns the necessary content.