The short answer: the summer before eleventh grade, for about eight to twelve weeks, with a second block before your second sitting.
The longer answer depends on where you are now.
By year
Ninth and tenth grade. Nothing formal, and resist anyone selling you otherwise. What helps at this stage is ordinary: read things that are harder than you find comfortable, and take the math course sequence seriously, because the SAT tests what you covered in class. If your school offers the PSAT, sit it and treat it as a free rehearsal.
Summer before eleventh grade. The highest-value block most students get. It is uninterrupted, it lands before the PSAT that counts for National Merit, and it means arriving at junior year with the test partly handled rather than looming.
Autumn of eleventh grade. The PSAT/NMSQT that counts. Sit a real SAT or ACT soon after, while the preparation is still warm.
Spring of eleventh grade. Your second sitting. Most students gain more between attempt one and attempt two than from any amount of extra study before attempt one, because the first sitting teaches you what the day is actually like.
Autumn of twelfth grade. A final attempt if you need one, early enough for early-action deadlines. After that the calendar decides for you.
How long preparation actually takes
Work backwards from the gap between where you are and where you want to be.
- Under 50 points. A few weeks. This size of gap is usually pacing and careless errors rather than content.
- 50 to 150 points. Eight to twelve weeks of consistent work. This is the normal range and where most real improvement happens.
- Over 200 points. A longer runway and probably two sittings. A gap this size means content gaps rather than polish, and content takes time.
Six to eight hours a week is the realistic cadence alongside school. More than that is usually a sign the schedule is about to collapse.
Why starting earlier is not automatically better
Preparation that starts a year out tends to go one of two ways. Either it becomes a low-intensity habit that never produces the concentrated practice that actually moves a score, or it burns out by month three and leaves the student sick of the test with the sitting still ahead.
Concentrated blocks work better than long thin ones. If you have a year, use two focused blocks with a gap, not fifty diluted weeks.
The first thing to do, whenever you start
Measure once, properly. A full-length timed test, or at minimum a diagnostic across every content area, before you study anything.
Studying before you know where your points are hiding is guessing, and it is the most common way students spend a month improving something that was never their problem.