Any time.
Next summer at the latest.
Get the Official SAT Study Guide and use it to prep for the PSAT.
Getting very familiar with the types of questions on the test is the key. Do not worry about having finished a particular math class. Just learn to do the problems on the test. (I do SAT/ACT prep. I have some 7th-9th graders going through practice tests. I also have 12th graders starting to prep for the Dec. test now…)