When Should I start revising for SAT Subject Tests?

I want to take the SAT Subject Test in Physics in May and I was wondering if that really takes much time to get ready for, or I should just start revision a couple of weeks before the test. P.S: I am familiar with the test’s content, but I do not feel really confident.
Thank you in advance.

bump

Take a practice test and see how much you need to learn to reach your target score. Plan your study time accordingly.

How much work is required to improve from 600 to 800?