Here is what the EPA hase to say about duct cleaning:
https://www.epa.gov/indoor-air-quality-iaq/should-you-have-air-ducts-your-home-cleaned
As for our own home, we have our ducts cleaned after construction/remodeling activity in our home and when we changed (years ago) from wood heat (airtight system) with warm air circulated through our ducts. Also, where we live in Texas, most people get their ducts cleaned when they buy a used residence because there is such prevalence of roaches and rodents, especially in seasonally occupied dwellings. My daughters both had theirs cleaned because cats, to which they are fiercely allergic, had resided in the homes.

Who cleans our ducts? Our furnace company, who we trust, does them here. They clean everything associated with the heating and cooling system every year (not ducts). After construction, they bring in power brushes and clean the ducts. All is sucked out with huge vacuum hoses into their truck and they have never left a trace of dust or debris. When they do it, they also check the seals on all exposed ducts. In Texas, one local company has a great reputation for being thorough and careful if not inexpensive.

So I don't believe in "routine" duct cleaning like the advertise. Between annual cleaning of the HVAC system, sealed ductwork, and my occasional vacuuming both the vents (remove covers and reach in with vacuum hose) and cold air returns it doesn't seem necessary. But there is a place for it. And if I had furry pets whose hair ends up everywhere, I would be likely to do it more often.
Sue