It used to be the case that auditors would avoid the last month, with the audit then undertaken sometime on the 4 months previous to the last month. This was the case when it was a 9 month window.
The unannounced window has now changed to 4 months for all schemes, and the audit can happen any time during those 4 months other than on days which the company have provided valid reasons to black out (up to 10) or on days where production does not happen (bank holidays, weekend, any declared non-production days). Certification bodies do not have a typical pattern that is followed, so any attempt to predict would be futile.
There is also some misunderstanding regarding what is being described as the 30 minute rule. Auditors are required to enter the production area as quickly as possible, with 30 minutes being the hard limit and not a target. In most cases I expect auditors to be in production within 15 to 20 minutes.