If the system is sealed air-tight and a really good filtering system is in place then you should NEVER have to clean your ductwork. Most dust people see in their homes (on beds furniture etc....) is broken down insulation that is being blown into the home from the attic (venturi effect) where the duct bucket (box etc...) is not sealed at the sheetrock ceiling. On the return side, ANY leaks in the ductwork will pull hot attic air and dust into the system, clogging up everything.
I took off stars for two reasons. Minus one star for issues trying to use the $50 for $25 voucher I purchased through Yelp. Although there was no minimum stipulated, the office wouldn't take it for the initial $59 diagnostic fee. If I used the 10% off First service that they offer, then they won't let me use the voucher on the actual repair bill. I was stuck with paying the full $59 diagnostic fee.
"Our Pro was the most courteous and informative gentleman that we have had the pleasure to work with, was great. He let us know immediately what the problem was and what the cost to fix it would be. Also, he let us know how to maintain and extend the life of our washer. He was very personable and made us very comfortable with the process. We would have him back in a heartbeat if anything ever goes wrong again. It was the best experience we have ever had with a appliance repair service."