when am I supposed to change a timing belt for a 2000 c280 ? thank you. also when I change it is there any other service I should do when I have it off?

Your car does not use a timing belt, it uses a timing chain. There is no set time/mileage limit for changing a timing chain but there is a procedure for checking the chain for stretch or wear.

Use the search feature and you will find endless threads that discuss timing chains, they seem to be a close second to oil thread as the favorite topic of discussion around here.

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98 Dodge-Cummins pickup (123k)
13 GLK250 (135k)
06 E320CDI (323K)
16 C300 (62K)
82 300GD Gelaendewagen (54K)

oh I didn't realize that thanks, I do see a belt, when do I need to change the fan belt?

I shine light on the inside of the belt and look for wear (cracking). As soon as I see any, I replace the belt. Otherwise replace about every 60K miles just to be safe, but I would inspect it occasionally.

-GH