I have a '98 e300. Changed the shut off valve, fuel filters, and plastic lines. (When you install the fuel filters, they should be filled with diesel fuel.) I had to crank the starter for about a minute and a half.

Cranking should be done in intervals of 20 to 25 seconds. Crank for 20 to 25 second, let the car sit for a few minutes, then crank again, then let the car sit for a few minutes, and repeat until the car starts. I think that is the way most people do it. You don't want to overheat the starter.