The need to change a filter depends on where and how the air purifier is used and the rate of new particle introduction into the room environment. Some manufacturers claim their filters last for years. But in reality, most filters lose their original effectiveness after 6 to 12 months of use. The filters for Hunter’s QuietFlo True HEPA models are designed to last for at least one full year under constant use. The filters for Hunter’s HEPAtech models are designed to last 4 to 6 months under constant use.