There's no hard-and-fast rule, but the more time students spend on Khan Academy, the more likely they'll feel well prepared for the SAT.
We recommend that students create their Khan Academy account in the fall of their junior year. If they've taken the PSAT/NMSQT, they can use those scores to get personalized practice for the SAT. If they haven’t taken the PSAT/NMSQT, they can take a few short quizzes to find out what skills they need to practice. Then they’ll have several months to practice before taking the SAT in spring of junior year.
Help your child create a practice schedule on Official SAT Practice on Khan Academy and stick to it. Like sports or music, the key to feeling prepared is committing to practice.