This is something that can vary drastically. Factors like how sensitive you are to allergies will drastically change our answer. If you have inhalers and bad allergies this may need to be done annually. If you don’t have allergies, and don’t have lots of animals then getting your ductwork cleaned may not be required as often. Asking your technician to asses your system ductwork and explaining to them your sensitivity to allergies is probably best. Then we can make an educated decision on your system duct cleaning needs.