If everyone in your house is healthy, you don't have to clean ducts regularly. There's no proof that it prevents problems, and done incorrectly, it could cause them. Dirt inside usually sticks and stays put. Vacuum or take off and clean grates when you notice normal dusty buildup. However, if you have mold, rodents, or insects, or you can see stuff blowing out of your vents, it's time to do something.