Manufacturers recommend that HEPA filters be changed at least once per year to maintain proper operation.

Yes.  Each instrument is calibrated annually and the documentation kept on file.

CO2 sensors are accurate as long as they are calibrated once per year.  T/C sensors can drift as much as 1% within one year.

The industry standard is one year.

Manufacturers recommend that filters and condensers be cleaned at a minimum of every 3-6 months.