It can be up there for more than 50, they probably won't work then, but they will still be up there, because it's so hard for them to get them down, it's easier just to send up a new one.
But, how many years it can there? can it stay for more than 50 years? does it have that much life in it?
Basically until they fall out of orbit?
A Comprehensive Guide on Calibrating Oxygen Sensors