Planning hours for externally engaged audits and Examination?

I am working to create our Internal Audit Department after being the monitoring body at my CU
For your annual audit plans, how do you account for hours spent providing support for External and outsourced audits and exams? Do you have a set time set for planning and then use a fraction for the onsite time spent?

I want to fairly represent the time needed to do these.

Thanks for your time.