We recommend 1-3 months. This number depends on a variety of factors: how many pets do you have, how old are your windows and doors, and other contaminants that are present in your home. Check your filter frequently to determine if it’s dirty. If it is, then you should change it. We never recommend going over three months before changing air filters. The more often air filters are changed, the more you are able to increase the lifespan on your unit.