Depends on how much studying you did throughout residency and how well you did on the inservice exams. I felt those were very similar in terms of content, particularly testable topics.
If you went through residency reading up on the core knowledge and did well without too much effort on the in service exams, you will probably do fine with relatively little prep.
If you slacked off, never studied, and didn't do well on the inservices then you probably need to do a pretty intensive prep in that month. Would probably take a board review course and do lots of practice questions.
If you are somewhere in between you will probably be fine if you do the last couple of PEERs. Assuming you've been a reasonably skilled test takes in your career so fare.
Either way, you definitely want to take it seriously. Apart from the expense, if you fail the boards your program director gets an email and you will be added to the wall of shame.