In my case, nope, never had them cleaned, have no intention of ever having them cleaned.

My house is a nightmare anyways.

I rent an old house that's had who knows how much unlicensed work done by different homeowners over the years. Just in my lifetime the house has had 10 different owners, that I know of anyways, and several of those owners rented the house out to people who seemed determined to destroy it.

The furnace is fortunate to even be limping along working at all it's so old and rusted, at this point I have a laundry basket proping up the panel on the front of it so it will draw air through the filter instead of directly into the furnace.

It's located in my basement which has constant water leaks all the time and mold all over the place. Even getting a filter on the thing takes a pair of scissors and a lot of duct tape. You have to cut the filter down to size since it's non standard and tape it on the front of a badly sealed metal box mounted to the side of the furnace. That's assuming the basement doesn't flood high enough to reach the filter which is within an inch of the floor itself.

One duct runs through a cement block wall in the basement, under my front porch, and to a register there. Someone didn't cut the holes to line up right so it's got a huge air gap where the parts don't line up right and often blows just cold air so I have to shut the vent. To boot they cut the hole too big in the cement and to seal it up stuffed a pair of painters pants (yes you read that right) around the duct work to keep the air/dust out of the basement.

I vacuume out the registers as far as I can reach about twice a year just to keep it from throwing dust and stuff on my carpets.

If I had a real furnace, and a normal house though, I'd probably have the duct work cleaned every few years.