How often you clean depends on a number of things, not the least of which is what the homeowner wants.
Some of the things that might make a homeowner think about cleaning more often are:

Smokers in the home, pets that shed a lot of hair and dander, water damage to the home or HVAC system, and people with allergies or asthma who might benefit from a reduction in the amount of indoor air pollutants in the home’s HVAC system after renovations or remodelling before moving in.

1. There are smokers in the house
2. pets with a lot of hair and dander to shed
3. The water could get into the house or damage the HVAC system.
4. Residents with allergies or asthma who could benefit from less indoor air pollution in the home’s HVAC system
5. after home renovations or remodelling
6. before moving into a new home.

Some companies use antimicrobial chemicals on the inside of air ducts to kill microorganisms like mould, viruses, or bacteria that have grown there.
Before using any chemicals that kill germs, the system should be cleaned well.
Any antimicrobial treatment you use in your HVAC system must be approved by the U.S. Environmental Protection Agency for use in HVAC systems only.
Antimicrobial chemicals are an extra service that are not usually part of cleaning air ducts.
For more information, look at the NADCA White Paper on Chemical Applications in HVAC Systems.

The U.S. Environmental Protection Agency says that “duct cleaning services usually cost between $300 and $1,000 per heating and cooling system,” but this isn’t always the case. It depends on the services offered, the size of the system to be cleaned, how easy it is to get to, where you live, how dirty it is, and what kind of duct material it is.

People should be wary of air duct cleaning companies that make broad claims about the health benefits of cleaning ducts, because these claims aren’t backed up by evidence.

Air duct cleaning companies that just “blow and go” should also be avoided.
Most of the time, these companies clean the heating and cooling system poorly for a small fee.
These businesses may also try to get customers to pay for services they don’t need, with or without their permission.
(If you know of a company that does “blow-and-go” air duct cleaning, you should contact your local Better Business Bureau to report the company and your local, state, and federal elected officials to ask for laws to be changed.)