You can stretch them a lot longer than 30K by 1) regular driving, and/or 2) periodic removal and cleaning of the contacts. If you don't do enough of either, the MB recommended 30K replacement interval works wonders.
1972 280SE 4.5
1984 300SD, 250K
1991 300SE, 89K
1992 300D (330K miles when KIA'd by mother-in-law 8/10/09 - still sitting in my barn)
1994 E320, 109K
1995 E420, 60K
Proud Member of the Mercedes Benz Club of America - Idaho Chapter