When you feel as if you are ready. Don't rush it, just make sure you have reviewed prior to taking it. Once you have completed Biology I & II, Organic chemistry, General Chemistry and Calculus, you will be more than ready. The more classes you have taken, the better your chances. If you haven't completed all the classes, just make sure you get a good study guide and begin reviewing approximately two months before the exam. Just my opinion. Good luck. You will probably get better feedback by searching through the PCAT section.