The answer
Semi-annual protocol (Spring AC, Autumn Heating). Multi-point inspection includes: chemically washing outdoor coils, clearing condensate drains, testing capacitor microfarads, checking contactors, calculating superheat/subcooling refrigerant charge, and checking furnace heat exchangers and combustion efficiency. Preserves warranties and recovers 5%-15% operating efficiency.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.