Firstly, there are certain key points in an audit plan which are somewhat fixed, for example your registrar visits. These should be shown on a calendar and you could choose to audit those requirements that the CB auditor will look at, plus any open/closed CARs.
Further, there may be operations which are tied to a schedule, let's say vacations - where a proportion of the workforce goes on holiday and you use contract labor for that period. That would be another great opportunity for a scheduled audit, to see what effect those 'new folks' ahve on the process, training etc.
After that, I have (in the past) planned to do audits monthly, but only choose the scope and criteria based on the issues at that moment - let's say 30 days before), to be flexible. I rarely use a crystal ball to see what needs auditing in 6 - 9, or even 12 month's time.....And it's never been an issue with any CB auditor!
Does this help you?