https://rainbowrestores.com/blog/how-often-should-i-clean-my-air-ducts

The most important details in this text are that homeowners should clean their air ducts at least once every five years, and that the service frequency depends on several factors, including climate, health conditions and pets. To determine how often to clean the ducts, homeowners should examine the filter or duct wall when replacing their HVAC air filters every two to three months, and shine a flashlight down the ducts when replacing them. Additionally, homeowners should replace their HVAC air filters every two to three months if they recently moved into a home or apartment without any air-duct-cleaning service history. The most important details in this text are the signs of poor indoor air quality, such as new allergy symptoms or respiratory problems, dust build-up around the house, signs of rodents or pests in the air ducts, and an inexplicable energy bill increase. Professional air duct cleaning is most thorough when left to the experts, and Aire Serv's air-quality cleaning checklist can help maintain healthy air quality outside of the ducts.