Everyone should get the air ducts swept out every couple of years or so, depending on what kind of air quality your house naturally has.
Some houses are so well ventilated that the air circulation system goes for a very long time before getting dirty.
In homes with a lot of animals or kids, or in homes that have little ventilation and rely on recycled air, the ducts can get dangerously dingy in a matter of months. The only way to be sure is to stay vigilant and check out your system every once in a while. You don’t have to be an expert to take the grate off of your air vent. You can shine a flashlight into the air duct and just take a look. If you can see dust, dirt, dander, or hair on the inside of the ductwork, that means it is time to call an expert. I advise you to never try and clean the ductwork yourself, you really need to have a professional do it for you. Air ducts are made from sheet metal, which is incredibly durable, but still can get weak and brittle over time. If you have old and weak ductwork, your attempts to scrub them clean might result in broken seals. If the sealant holding the ductwork together is compromised, your air quality suffers and the overall performance of your HVAC system becomes weaker. If you know what you’re doing then by all means clean your own air ducts. Otherwise, I think you should leave it to the HVAC professionals.