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.

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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