It depends on how hard the classes are and the workload that comes with it. I did 21hrs. You can go ahead and sign up for these classes and attend first day just to get the syllabus and see what is expected for the semester. If it's too much then drop the first day and you will get a full refund. Good luck!
It depends on the difficulty of the classes, how hard you're willing to work, and what type of student you are. Do you have to study quite a bit, or can you read something once or listen to a lecture and remember the information?