There is no precise schedule for using duct cleaning services since each home has a unique enviroment. Normally it is a good idea to have your home cleaned once every 2 years to eliminate the buildup of dust and dirt. The longer you wait the more pollutants you will be living with. All the dust that circulates through your home goes through your air ducts to your furnace and back again. If your home as a lot of house spiders, their webs can capture dust and aggravate allergies. As dust and dirt builds up, pieces break free and float around the house until you breath them in. Pets that shed hair can cause dust balls in your walls giving a place for dust mites and other bugs to live and nest.