The short answer
Earlier than the spring of junior year, which is the standard school advice and is late enough to collide with college applications. Plan two sittings rather than one, since nearly every college superscores, and give each sitting a dedicated four-to-eight-week sprint — five or six weeks at around ten hours a week is the middle of the range. A student still taking Algebra II doesn't have to wait either: English and Reading don't touch it, so they can sprint on those first and come back for Math a semester later.
Timing is the cheapest part of ACT prep to get right and one of the most common places families lose points, because the default schedule comes from somewhere with no stake in your student’s score.
Why “spring of junior year” is the wrong default
That’s the timeline most schools hand out, and it’s late. It leaves one realistic sitting, pushes any retest into the summer before senior year, and lands the whole thing on top of the college application season — essays, deadlines, and prep competing for the same evenings.
The in-school test date deserves the same skepticism. States contract with one testing company to administer a test on a set day, so the date on your student’s school calendar reflects that contract rather than your student’s schedule. It’s fine as a data point. It is not a plan.
The other version of this we’d steer you away from: registering for a real test just to find out where your student stands. A realistic practice test, taken at home with a real scoring key, gives you the same number for free, and it doesn’t put a score on a record.
Plan two sittings, on purpose
Most colleges superscore, meaning they take each section’s best result across all sittings and combine them. That changes what a test date is for.
With one sitting, a student has to be good at everything on the same morning. With two, the first can be aimed at two or three sections and the second at whatever’s left — often a single section by then. Concentrated prep on one section beats divided prep across three, so the same student scores higher on each piece, and the superscore collects the best of both. Boosted Brains almost always builds toward two test dates for exactly this reason. Brown kept going past two and rode a 31 up to a 34 across later sittings; her Auburn award moved from $44,000 to $60,000 with it.
How long each sprint needs
Count backward from the test date. Five or six weeks of focused work at roughly ten hours a week is the middle of the range and where most students land. Compress it to three or four weeks and the weekly load climbs to fifteen or more. Stretch it to eight and six to eight hours a week is enough.
What matters more than the exact number is picking weeks that are actually free. If baseball season owns April, the April test is the wrong test — not because the prep is impossible but because it will be the thing that gives. Ryder started in September of his junior year and finished at a 33 during football season, and his classmates thought September was absurdly early. He spent his senior fall playing football while they were still testing.
The short version
Find two stretches of five or six clear weeks on next year’s calendar, register for a test date at the end of each, and ignore the schedule somebody else set. For most students the window opens in sophomore year.
One more case: if your student is still taking Algebra II
This applies to a minority of students, and it changes the order of the work rather than the start date.
ACT Math leans hardest on Algebra II and stops short of calculus, so a student who hasn’t taken it can’t max out that section yet. The common conclusion — wait until it’s done — is the wrong one. It usually pushes the start to the spring of junior year, and paying somebody to teach Algebra II content that a high school is about to teach anyway is a poor trade.
Split the sections across the two sittings instead. English and Reading don’t touch Algebra II: English tests grammar and sentence construction, which most high school English classes don’t cover at all, and Reading is comprehension under a clock. A student can bank strong scores in both while Algebra II is still in progress, then come back a semester later for a sprint aimed entirely at Math. Superscoring assembles the two.
The optional Science section doesn’t gate anything either. It’s data interpretation rather than science knowledge — only two or three of its questions touch actual content, at the level of water’s boiling point. Waiting on a chemistry class buys nothing.
So where the math sits determines what the first sprint aims at, not whether there is one.
