Oh goody! Another oil thread!
(cough)
To add to what Brian wrote, every engine will produce a different amount of soot, and your driving habits affect this also. The only way to know actual soot produciton rate on YOUR engine is to get an analysis. In general, the OM61x engines seem to make a lot more soot than the OM60x, and all indirect injection engines will make more than direct injection (i.e., Cummins, etc).
There are two main limits to consider. First, the amount of soot the oil can suspend (about 2% for most Mobil-1 varieties, and about 4% for Delvac-1 and M-1 5W-40 "SUV formula"). The other limit is that (allegedly) MB has stated engines with hydraulic lifters - meaning all OM60x - should not exceed 2% soot load, even if the oil can suspend more than that. You could end up needing to change at 5 or 7.5k, or on the flip side you could go out to 15k+, depending on YOUR engine and driving habits! I have tested all mine at each oil change and found that 10k works well for my OM60x, and about 7.5k on the OM61x - for MY engines, and MY driving habits.
I use Delvac AccuTrack OA kits from Snider. The "extended drain" kit is required to give the soot %, they are $12.50 each, plus S&H to you, plus S&H to mail in the sample... about $18-$20 total per sample:
http://www.sniderpetroleum.com/oilanalysis.html
Regards,