The electrode wears out, it is a function of time, as well as how well your engine runs. If you're too lean, they get too hot. If your engine is too rich, they foul out. Thats also a function of whether or not you have the right heat range. If you're running the OEM plug in an OEM running car, they should last quite while. But if its like my truck and takes Champion plugs, they get replaced whenever they are on $.99. The 3800SC which takes $9 iridiums gets them less frequently lol.
Before someone else says it, this question is pointless, as real Mercedes don't have spark plugs.
