I would say start prepping after finishing Algebra 2, but also when you and your student feel s/he is ready. Go earlier if you think you’ll want multiple sittings, if you’re trying for selective schools and/or need a certain score for financial aid.
To us, high school doesn’t seem like it will get any less busy or less difficult, so the summer between sophomore and junior year seems a reasonable time to squeeze in a test prep class. Your schedule may vary, depending on what math class your student is in and his/her readiness.