This is very much dependent on the occupants of the house and the setup of the house. The number of people living in the house, the number of pets, the amount of carpet, the level of activity, etc all contribute to the amount of dust within the house. This household dust then gets draw into the ductwork. Average household maintenance duct cleaning is recommended every 3-5 years. However, some clients with severe allergies find that cleaning once per year is necessary for their wellbeing.