Hi Arial, good question! I would recommend to study about a year before you apply to medical school. The application process is different; for example if you want to go to medical school after graduating college without any wait, then I would suggest to take the MCAT your junior year around spring such as March/April maybe early May, then apply in June to medical school. Your senior year in college will consist of hearing back from medical schools about interviews and hopefully you will gain an acceptance before you graduate and then you will begin shortly after graduation. I also agree with Richard's comment about retakes, so take the exam early spring if you are not satisfied then don't apply and re-study and re-take when you are ready. That being said you also want to be prepared for this exam so take all the classes you can regarding the prerequisites which will be tested heavily on the MCAT and recommended courses that will help in preparing you better. Give yourself time to study, usually about 400-500 hours of prep time before exam day because the test has been revised and it now includes psychology, sociology and biochemistry as well- know those amino acids! You should maybe study about 3-4 months, that could change though especially if you are in college as a junior with other classes and if you have other responsibilities, some individuals spend 6 months to even a year. It's important to just stay dedicated though and not give yourself so much time to where you ease up on studying, so maybe setting a test date and getting that appointment for your date will motivate you to absolutely be ready. Make sure you do well in your classes though and keep volunteering and shadowing so you can be prepared overall as an applicant!
P.S. Check out AAMC.org and Khan academy, they give great advice for what the MCAT is, in addition AAMC.org is where you begin your application to medical school and there's a ton of information for premed students!
Best of luck!