The short answer
The National Air Duct Cleaners Association suggests every three to seven years for a typical home. That is a wide range because the honest answer depends on your house, not on a calendar.
If someone tells you every year, they are selling, not advising.
What the EPA actually says
This is worth quoting directly, because it cuts against the way this service is usually sold. The US Environmental Protection Agency's position is that duct cleaning has never been shown to prevent health problems, and that particle levels in homes do not necessarily rise because of dirty ducts.
So if a company tells you that cleaning your ducts will help a child's asthma or your allergies, they are making a claim no regulator supports. We do not make it, and you should be wary of anyone who does.
The three cases where it is genuinely worth doing
Visible mold growth on the inside of hard surface ducts or on other components of your system. Not dust — mold. If you can see it, it needs addressing, and the source of the moisture needs finding too.
Vermin infestation. If there is evidence that rodents or insects have been living in the ductwork, the ducts need cleaning and the entry point needs sealing.
Debris that is actually restricting airflow. Ducts clogged with excessive amounts of dust and debris, or where particles are visibly being released into the home through the registers. Renovation dust is the most common cause we see: drywall sanding puts a fine, heavy load into a system that was never designed to filter it.
What is not a reason
A thin film of dust on the register grille is normal and tells you nothing about the duct behind it. Dust settling on furniture a day after cleaning is usually a filter problem, a leaky return, or simply a dry, dusty climate — none of which duct cleaning fixes.
Before you book anything
Change the filter and check its rating. Look at the return air grille for a gap where unfiltered air is being pulled from a wall cavity or attic. Those two things solve a surprising share of the calls we get, and both cost less than a service visit.
If you have done those and the problem persists, a technician can put a camera into the trunk line and show you what is actually in there before you commit to anything.