It depends on the exam, score you are aiming at and how busy your schedule is. We recommend start preparing 12 months to 2 months ahead of the official exam date: SAT and ACT (at least 2 months), SAT Subject Tests (at least 2 months), AP Exams (at least 6 months) and Olympiad Exams (at least 12 months). Note that any amount of prepping is better than not prepping at all.
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