How often should I replace the True HEPA filter and the Odor Reducing Pre-Filter?

The True HEPA filters should be replaced every 12 months, and the Odor Reducing Pre-Filter should be replaced every 3 months. Replacement may need to be more frequent in homes with heavy odors, dust, pet dander, or smoking. These intervals are guidelines only, as performance depends on the concentration of contaminants in the environment. (Page 3)