When I was in A&P, I did not study specifically for tests, per se. I studied 1-2 hours every day, from the first day of the semester up until the final exam (there were of course times I took a day or 2 off). That prevented me from having to cram before exams, and I seemed to know and retain the information much better.
15-20 hours is a lot to study, especially if it is 1-2 weeks before the exam. But, like they say, "if it ain't broke, don't fix it." If you are comfortable studying this much and do well on your exams, I don't see that you have any reason to worry about anything.
We are all different and thus learn at different speeds.