Depending on how often you use the climate control, I would say somewhere in the neighborhood of every 4-5 years would be a good replacement schedule. It doesn't have to be replaced nearly as often as the dust filter.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record