Dealt with in more detail in this thread on here.
OM651 timing chain @76k miles, to replace or not?
Early versions of the engine appear to have had problems but resolved in later versions with improved components such as chain tensioners. Early warning signs of timing chain noise should prompt action but otherwise regular oil and filter changes with good quality oil of the recommended viscosity should ensure a long life. One thing to bear in mind is that timing gear wear is governed by several things, not just mileage. An engine running fairly constantly at revs well below its limit is going to place less strain on its timing gear than one undergoing multiple stop/starts or constantly having the nuts revved off it!