Iso 14644-3.pdf Better -

Since you are referring to , titled "Cleanrooms and associated controlled environments – Part 3: Test methods," it is indeed a fascinating document, though perhaps "interesting" is a polite way to describe its complexity.

The (Section 5.0) offers a "decision tree" for frequency:

All instruments used (particle counters, anemometers, etc.) must be regularly calibrated and traceable to national standards.