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?