That is the hardest part of the question. Of course, if they suddenly stop working because of a fault, you'll immediately know.
And if you upgrade your computer, and the device simply doesn't work for you any more, you'll know (although of course you'd go to the calibrator's website to see if there was an upgrade or patch.)
And if you upgrade your screen to something fancy and new, and suddenly the calibrator gives you outrageous results, you'll know (although of course if that happens you'd post here for me so we could troubleshoot.)
BUT ...
What if it begins to die very slowly or subtly? Just a small shift in the results it produces, over time? That's the worst, because the first time you're aware of the problem might be when you open an expensive delivery of prints from your lab, which are all slightly too pink, or something.
So keep your original test prints safe in a dark place, and compare them to the screen after each calibration, or at least every second calibration, once the calibrator is a few years old.