MCAT有效期3年。有的医学院只要2年。

http://www.gradschoolhub.com/faqs/how-long-are-mcat-scores-valid/

If You Earn a Stellar Score, How Long Will It Be Valid?

Preparation is extremely important when you are planning to take the MCAT. Since your score could make a difference in acceptance to med school, you need to allow extensive time for preparation and study; the MCAT content is not something you can cram right before the test. If you decide that you want to take the MCAT and apply for school at a later, you should be aware that your scores are only valid for three years. Some schools even limit scores to within two years of application date. If life happens and you are not able to apply for medical school within that timeframe, you must retake the MCAT exam.

https://students-residents.aamc.org/applying-medical-school/faq/mcat-faqs/

When should I take the MCAT exam?

In most cases, you should take the MCAT exam in the calendar year prior to the year in which you plan to enter medical school. For example, if you are applying in 2016 for entrance to medical school in 2017, you should take the exam in 2016).