I see no written in stone mileage change time on them. Some owners say 12,000, some say 18,000, some never change them. The dealer price is obscene and should should be investigated. I ordered one fom Amazon and it was $27.80 including shipping with no state tax. I also bought one from Auto Zone for $35.00, no shipping but 8.25% sales tax.
The one I changed looked good, but after a week or so the leak appeared. The two new ones and the old original were all manufactured in the same plant in Germany, with diff names stamped on the bodies. I am going to order two more from Amazon at that price now that I have two cars, but the same part # is listed for more than 100 models Mercedes. It's only a 5-10 minute job to change it and a couple of pieces of 2x10s to drive the car on to raise it a little. Be sure to turn the steering wheel a few inches to clear the pitman arm


John P