Good question. I suspect a very long time if it is operated within the design range and not physically damaged. The coil spring on your BK's stat is the same thing. The finish of the meter face or the probe rod itself may fail first.
The literature that came with my Condor said one year. Mine is near the end of its second full year and still doing fine. I suspect the higher your average flue temp, the sooner you need to replace the probe.