It is hard to say on the lifespan. Since the measure criteria I have given is strictly subjective and MB doesn't give much in the way of operational criteria, I would replace the t-stat as often as I would replace the other t-stat for the radiator. The wax bulb that operates the two are very similar.
The criteria I gave for determining how far the valve has shut does a pretty good job as long as nobody has messed with it. In other words if the air valve makes it shut when hot, the t-stat is probably working as the thing fails in the rich direction. The wax bulb grows as it gets hot and drives the airslide down shutting off aux air and reducing the cold running enrichment with the same movement. Since the device has two different sets of shims within, the airflow method can be messed with giving different characteristics with the t-stat working properly.
Bosch Master, ASE Master, L1
33 years MB technician