We started to think about options after getting the sophomore PSAT score.
For my kid with a fairly high PSAT score (high 1300s), we encouraged him to use Khan Academy the summer before junior year (which he barely did). He took the SAT and ACT in the fall of junior year, was happy with his scores and was done. (he was in honors pre-calc junior year).
For my kid with a fairly low PSAT score (1100), we signed her up for a group SAT prep class for the winter of junior year (she was in regular pre-calc and didn't have all the required math before then). The class didn't help much, her SAT scores were no better, but she ended up doing well on the ACT with some additional math prep from a book. She took her tests in the spring of junior year (two SATs and two ACTS - in hindsight, we should have had her try an ACT much sooner).