Many Holmes air purifiers include a filter indicator light or digital display to take the guesswork out of when filter replacement is needed. Your Holmes HEPA filter needs to be replaced every 12-18 months depending on use, while the carbon prefilters should be replaced every 3-6 months.
Holmes humidifier filters should ideally be replaced every 2-4 months, depending on use and water hardness. Harder water will decrease the usable life of your wicks. You should always discard your used wick filter at the end of the season, prior to storing Holmes humidifiers.