I need to do my dust filters every 15,000 miles or less. They are cheap and just get too dirty to leave in for the recommended 30,000 mile interval. Charcoal I replaced at 50,000 miles per the manual, but since I rarely use the charcoal filter, I'll go quite a bit longer this next time. They run about $100.

Len
'99 E300TD 89,000 miles