I believe that Mercedes recommends replacing the charcoal filter every 60,000 miles at a cost of around $200.

I rarely activate the charcoal filter so I have not changed mine yet. The Parts person at the local Mercedes dealer suggested that I replace my charcoal filter (at 60,000 mi.) otherwise I will risk getting bacteria in the car. I did not buy this because by his logic every Mercedes without the charcoal filter must have bacteria in it.

I will replace my charcoal filter when it can no longer get the smell of skunk from the car. No kidding, when I approached a dead skunk on the road, I activated the charcoal filter and within 5 sec. I no longer smelled skunk (note the windows and sunroof must be closed for this to work).

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles