Simply, the task before you is to provide and assure clean air. So there are a lot of things mixed below:
How you provide clean air and suitable environment:
air changes per hour
How you verify suitable environment is maintained:
counting of particles
differential pressure indicator
So, you start with a properly engineered room with gown room. Depending on the class the room was designed for, there should be sufficient filters and adequate air changes per minute/hour.
As long as the air flow is suitable, filters work like they should, environmental system is working fine, positive air pressure is maintained, and clean room guidelines are followed, the air should be clean. The appropriate parameters are then verified with the particle counters, differential indicator, and temp/RH monitoring.
Calibration of the counters, indicator and temp/RH indicators should follow an analysis of the instrument performance. Usually I would recommend once or twice a year. They can be performed when there is a planned clean-room shutdown.
Filter maintenance can depend on a lot of factors including time since last change, how much filters are moved around and such, and cleanliness of the incoming air.
If you have continuous particle monitoring, I would perform calibration and maintenance every six months or a year, unless you can find an industry requirement that would supercede that. If you're having alarms, lose positive pressure, etc., you might have to perform it more frequently.