I always buy the top-of-the-line 3M filters. Expensive and yes, they show the dirt they filter out of the air.
I check it from time-to-time and replace when the white filter turns grey. Spring, middle of Summer, and Fall. As the heat don't go on much here in the Winter it's good from Fall to next Spring.
Walmart, Target, Home Depot...all carry them but may be out of the really good (read $$$) ones. Shop around, look for cupons, etc.
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