If you keep the a/c filters changed on regular basis, I see no need in having ducts cleaned. However, if the ducts have come apart in the attic any place, then, dirt, etc can get into the ducts.

I have lived in our house since 1965 and have never had ducts cleaned.

Is the ceiling around the vents clean or have dirty look around them? If ceiling is dirty then ducts are not clean and/or ac has a problem, or mold has formed due to a leak from ac.... if ac is in the attic.

One way to test and see if ducts are dirty, put a/c filter fiber over vents and see if they become dirty. The filter fiber looks like quilt batting but is intended for ac vents. Not sure whether polyester quilt batting would put a strain on the ac or not.

Only time I used filter fiber was when we bought allergy special filters and the mfg. sent vent fiber to put over vents before using allergy filters. The fiber did not get dirty.

duct cleaning is more over rated than miele vacuums

Most people are now aware that indoor air pollution is an issue of growing concern and increased visibility. Many companies are marketing products and services intended to improve the quality of your indoor air. You have probably seen an advertisement, received a coupon in the mail, or been approached directly by a company offering to clean your air ducts as a means of improving your home's indoor air quality. These services typically � but not always � range in cost from $450 to $1,000 per heating and cooling system, depending on the services offered, the size of the system to be cleaned, system accessibility, climatic region, and level of contamination.