The only way to determine max drain interval is with oil analysis. Every engine produces different amounts of soot and other contamanints (sp?). On my 1987 engines, analysis shows I could probably change every 15-20kmi safely with Delvac-1. On my 1984, more like 10-15kmi because it produces more soot. This assumes that soot is the limit (which it usually is), and that TBN is ok, and other wear metals are normal. I usually change at 10kmi because that's 12-24 months of driving for me, and it's an easy number to remember (change at 230k, 240k, 250k, etc).
FWIW, Doc Booth (MBCA diesel tech advisor) mentioned that the OM60x engines with hydraulic lifters should have the oil changed when the soot reaches 2%, even if the oil used can suspend more (i.e., Delvac). I'm not sure where that recommendation came from (MB or elsewhere) or what the exact reason is. Supposedly this doesn't apply to the older OM61x engines, those can go right up to whatever the oil's limit is.
I scanned some of my analysis reports, and posted them in PDF format here (scroll down to "oil...") :
http://www.meimann.com/docs/mercedes/
HTH,