It's really up to you. The Blitzes typically take 1-2 months to complete and we give 9 months of access. So if you purchase today, you'll have access until around Dec 26, 2022 and you'll want to test before the end of the year to get it out of the way.
You can even watch the Blitz alongside your current class. Every time you're working on a similar topic/chapter, you can also watch the relevant Blitz module and mark up the respective chapter in your CPT manual following our BHAT® method. It helps strengthen your knowledge and retention.