Having done a few hours/miles with diesels, I'll politely try to answer the question.

First of all, if the engine has a history of clean and dry fuel then the injectors will last a very long time. The old trucks (pre-"low sulfur" fuel) would go about 400,000 miles, modern trucks can go to a top-end overhaul (1,000,000 miles thanks to good lube oils) if fuel additives are properly used. Marine diesels operate on the clock and the hours depend on where in the world the fuel comes from. I don't know the answer for MB, but if the fuel is kept clean and dry then 250,000 miles is not out of the question. Nothing, including injectors, is impervious to wear.

Clean means clean. Dirt getting past the filters is an injector and injector pump killer. Premature wear from dirt will throw off the crack and opening pressures, especially in relation to the other injectors. You want them to operate by the book in relation to each other as well as to the absolute pressures. Dirt also disturbs the spray pattern.

Water is a tip killer. Water will flash to steam as the injector cracks, and can literally blow the tip off the injector. You'll notice this as a big puff of white smoke followed by the drippy injector symptoms of nailing and glow plug failure. Sitting with the tank partially filled is an invitation to condensation in the tank, especially as we enter the fall season of temperature swings.

Diesel isn't sterile: it contains microbes we commonly call algae. They need water to survive. When they die they turn into black slime that clogs filters. Also injectors and pumps if the engine sits and wasn't properly treated with biocide.

It takes the proper equipment to test, clean, and adjust injectors, and experience in judging when the job is done properly. Doing it now or going by your (or a TRUSTED mechanic's) judgement is up to your comfort level, but remember it's best to do them as a set, or all at the same time. In the meantime there's nothing wrong with being anal about clean fuel and tracking your fuel mileage by the tank. That's what the people who make a living with diesels do.

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daBenz - 1970 220D