How often are you suppose to replace the cabin charcoal filters? Is it based on mileage, years or usage?

I replaced mine when they ceased working - when they work they allow NO smells of exhaust or rain or grass cutting it's amazing. When they have absorbed all they'll take then you will start to smell the stinky cars in front of you. I have 2 years / 20k miles on the ones in the car now.
I know there is a reccommended replace interval but this is my experience; YMMV. Do a search, some techs have answered this before.
-Tom

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'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
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2004 Audi A8L
'98 VW Passat 1.8T 5M
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'67 Impala convertible, 327cid