That depends on your specific needs. It depends on if you smoke, have pets, or any other contributors to dust or allergens in your home. I think they recommend every couple of years though.
Cleaning frequency is dependent on the maintenance after the cleaning has been performed. If using a good air filter (MERV rating of 7), changed every 2 months that the HVAC system is in use, one can reasonably expect the longevity of the cleaning to be 3-5 years. Factors that will increase the frequency of cleanings would be in-home construction (drywall, carpet, hardwood floors), pets or other dust producing activities.
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