Depends on what you can handle. Personally, I have usually just taken 13 hours a semester. This is going to be my last semester. I will be taking:
Honors Investigation (research in a lab, about 9 hours a week)
Organic Chemistry Lab
Physics 2
Human Anatomy
Human Physiology
This is way more than I usually take. Anyway I don't know why I just gave you my schedule lol, but basically what I'm trying to say is that no schedule is too much if you can work hard enough.