Your ducts should be cleaned regularly, it is important to check your ducts yearly to make sure there isn’t any accumulation of debris in your vents. having your ducts cleaned will remove any contaminants like pet dander, dirt, dust and other allergens. If you have recently had an infestation in your walls or attic, have any visible mold in your home, or notice dust and/or dirt coming through your vents, then it is recommended that you clean your ducts ASAP.
Minor cleaning can be done on your own with a rag, brush, or vacuum cleaner to remove dirt or dust from the grilles and supply/return registers. If you are not comfortable doing so, or you need a whole house duct cleaning it is better to call a professional to clean the whole system.