Filter change Frequency?

I have some problems doing maintenance and when facing any internal audit. What is the cleaning and change frequency of G4, F6,7,8,9 Filters. What is the difference of AHU filters in HVAC system & Process machine AHU system like Coating or Granulation unit?

We have colored in DP Gauge (on AHU) green & red with our experience and trial error basis, but in audit period they auditor asking for operation and alarm zone on the DP gauge. What are the guidelines regarding this problem according to GMP/GEP or ISPE?

Contact your filter supplier for the data sheets of the filters, these should give you the expected change details.

Example data such as here..
http://www.airclean.co.uk/PDFData/Ca...%20Filters.pdf

We are using Camfill filters, in their manual they have all the relevant parameters but the problem is in actual practice the DP across the filter is never matched.
Suppose a F6 filter pressure drop is 250 Pa as per manual, but in the system we installed 10 filters then what will be our expected pressure drop across filters? If we want to put all the Air Handling Unit filtration unit under standard and optimized economical system maintenance what will be our protocol or SOP or guideline?