In daily use, with proper maintenance and so on, I would guess that life span of the quality locos in measured in decades. As for "real" continuous use (i.e. take it out of the box and run it until it stops), you are probably looking at a few hundred hours until lubrication is needed, which if not done, leads to drive train problems...
There was a guy posting at the MR forums who volunteers at teh Chicago Museum of Science and Industry, and he was giving regular updates on their new US$3.5 million layout, including wear and tear on the locos.
Hope that helps.
Andrew