Two considerations; how many hours of playtime (not how 'long' they've been on the guitar) and how were they played (pounding heavy metal or sweet folk finger picking?). And of course the environment, the manufacturer, string gauge, string type, yada, yada, yada.

It's gonna have to be an individual decision based on one's own experience.

There is one guy that has a fair sized presence on YouTube that said at least once that he "never" changes strings on his electric guitar. Of course, he also said he was buying guitars as an "investment" and as life insurance to leave to his wife. Tsk.