If you're working 10-20 per week, you might consider taking fewer class hours. Furthermore, it may depend on your major or the level of difficulty of the courses you take. 12 semester hours is considered full-time at Iowa, but many students try to average around 15 hours to keep them on the path to graduating in 4 years. 12 -16 credits is generally safe for most students. This would depend on the individual experience that each student brings to the college arena.
Some students adjust to college level course work more easily than others.
If you attended a competitive or challenging high school that required a great deal of studying, you may make a relatively smooth transition to college. There are many such factors that make college study time vary from student to student. It is generally recommended that you study at least 2 hours for every 1 hour of class time. For instance, if you are in classes 12 hours a week, you should spend at least 24 hours per week studying. If this formula does not produce the results you're aiming for, you may want to study more.