I think the manual calls for every 50,000 miles on my car. The manual also calls for 30,000 mile change intervals for the dust filters. I find that I need to change the dust filters every 15,000 miles. I rarely use the charcoal filter, so I suspect I'll go at least 50,000 miles and probably more before I replace it again.

Len
'99 E300TD 75,000 miles