It depends on the course, the professor, and even how good of a grade you want.

I believe the minimum to be accredited is 3 hours per credit. So that 4 credit class that meets for 3 hours a week should have 9 out-of-class hours. I've seen some places recommend 3 hours of outside study time per credit, period. So 12 hours per week.

How much you do depends on you. I'm a CS major and other than a handful of classes, I'm able to complete most homework/projects within 3-4 hours per week. For my math classes, more or less depending on the workload (Calc 1 and 2 had dozens of problems, Calc 3 usually has 5-8).

If I look at my schedule this semester (14 credits), I'm in class 12 hours a week (no labs) and go to at least 1 hour of office hours a week. No idea how much time I actually spend studying and working on assignments, but it's a fair bit.