Dirt in the ducts
Some ducts give you enough of a peek to see how clean they are. At that point, it’s subjectively up to you as to how dirty you want them to be. If there is approximately 1/16” layer or more of dust, you may want to consider having them cleaned.

Excess dust in air and on surfaces
Excess dust can mean that dust and dirt are blowing out of registers on a regular basis into our breathing air. In households where country settings or pets create dust, and especially if children keep it stirred up, there’s a need to monitor the dust inside the air duct system.

When your air handler is replaced
A new air handler generally will blow harder than your old one.  This extra velocity will dislodge dust and dirt that lay undisturbed by the weaker old fan. Plus, the process of attaching the new unit to the old ductwork will shake them uppretty good and release more debris for the new fan to blow out the registers and onto the carpet, furniture, etc. Air ducts should be cleaned whenever an air handler is replaced, and even when anew blower is installed.

Filter problems
If the filters have excessive dirt or holes in them, that’s evidence of extra dirt in the system. If a filter has been absent for some time, it’s a good idea to check for dirt buildup.

Evidence of mold
Mold itself, or dampness that can promote mold, requires quick attention. Otherwise mold spores can be spread and possibly cause health problems.

Odors
Although the ducts may simply be transporting odors (and not the actual source), that transporting can leave a duct “stained” with the odor. This is especially true of smoke smells. Where problem odors are present, a good duct cleaning with an anti-microbial treatment can’t guarantee to solve the problem, but it can help a lot.

Sickness
Mold spores are not the only contaminants that can cause sickness if distributed by the duct system. Many allergens can find their way into the ductwork and cause irritation and discomfort. Many people who are sensitive to these invaders say that duct cleaning helped lessen the discomfort levels.  Some dramatically.

(As paraphrased from an article written by Dan Stradford on March 2, 2005)