General Academic recommends that rising high school juniors start preparing in the summer (June/July) in the lead up to the fall test dates (August/ November). This timeline allows the student to work around particularly stressful periods like mid-term and final exams. And it provides ample time to prepare not only for the first, fall test date but also a second or third testing in the spring of their junior year. Ultimately, the goal is to have the student finished with their testing by the May test date as a sanity check and to leave the summer period open for their college applications and essays. However, the student will still have plenty of opportunities for more testing in June, August, October (latest for early decision), and November (recommended latest for regular decision), but this would be on top of their applications and regular schoolwork in the fall of their senior year.
The most valuable resource in test preparation is time. The most difficult math topics tested on the SAT and ACT are Algebra II and some trigonometry, which most students will have already learned by the start of their junior year. Therefore, in order to avoid regretting not having started early enough, the summer before junior year is the best time to start.
With that said, “test prep burnout” is a very real phenomena. Parents should avoid cramming too much test prep into an especially tight or stressful period of time, and they should accept if students are already scoring at their peak ability.