To the OP, apologies for the sarcasm. However, by this point in your career it should be clear that people learn differently and people have different funds of knowledge. Therefore, uniform recommendations are not useful. I've known people who studied for Oral Boards for one year and failed and one person who actually did two weeks and passed (with flying colors!).
Use your experiences with the USMLE I/II/III, Shelf exams in med school, and Rad Onc in-service exams to judge appropriate time for you to study,