Once a year you should get your HVAC worked on, but for me, I do a twice a year cleaning.
I get the A/C portion focused on in Springtime and the heating method in the fall.
I have a central heating and cooling unit though. So it is 1 system that gets the service on it; But for cooling, I get new refrigerant and the fan blades inspected. For heating it is all about the dust in the system. I entirely love getting the twice a year cleaning to ensure our air quality is fresh. I also want to remove all the hair that gets in our system. I have entirely long hair and I shed worse than any cat. I have to clean our dryer lint trap consistently. My tub drain needs to be snaked bi-weekly. I also vacuum and sweep to get the hair off our floors. An air filter should be able to last in a system for around many weeks. With our long hair, more than one to many weeks is about all it can take. So I figure that normal people go a year with 1 cleaning, I do more than one cleanings to get our hair removed from it. The air filter doesn’t stop everything from getting in the system. I don’t want to have our long hair ruining parts in the HVAC. I also get air duct cleaning way more than any normal homeowner does as well. I don’t want dust, debris and again, our hair clogging up the air ducts. The plan of breathing in our old, dusty hair sounds disgusting.