I currently have approximately 16,000 miles on OE plugs in my SL500. It still pulls strong with no indication of missing, etc.

At how many miles do most of you change out your plugs.

25 - 30 k miles would not be unreasonable.
One of the ways plugs will tell you to replace them, is when you gently pull away from a stop.
This is called " tip-in ".
If you get any misfires then, it is certainly time to change them.

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2007 C 230 Sport.