How much time should I spend studying for my online classes?
Per California Education Code - Guidelines for Section 55002.5

In this section, the basic unit of college credit is defined to avoid reference to specific term lengths (previously described in terms of hours/week over a 16-week term). A minimum of 48 hours on the semester system (or 33 hours on the quarter system) of lecture, study, lab work is required for one unit of credit regardless of term length. The section establishes the minimum expected time on task (lecture, study, and or lab work) that is necessary to award one unit of credit. In practice, the number of hours varies among institutions, but is generally within the range of 48-54 hours per unit for colleges on the semester system. For each hour of lecture, it is assumed that students will be required to spend an additional two hours of study outside of class. The number of units awarded for laboratory courses is generally based entirely on the number of hours of laboratory work, presuming that students complete most required work in class.

For online classes which are 3 units then you would spend a total of 9 hours per week for 15 weeks (or a total of 144 hours - calculated as follows: 9 hours per week (3 units = 3 hours in class (online time) plus 2 hours for each hour in class = total of 9 hours per week x 16 weeks). This surprises many students because they have a mistaken idea that online classes are much easier than on campus classes. To give you an idea, for every class online you would spend the following each week:

2 hours reading the textbook chapter assigned for the week
2 hours working on assignments
2 hours working on any quizzes
2 hours working on any class projects
1 hour for posting to class discussions
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total time = 9 hours study time per week per each online class (3 units)