Life span of selenium rectifiers?

[Jeffhs]

I have two Zenith radios, K731 and C845, both with selenium rectifiers in their power supplies. Both radios still sound great, no hum or other problems as yet. However, I'm wondering how long the selenium rectifiers in these sets are going to last. The one in my C845 is as old as the radio (50 years this year); the '731's is also the radio's age (47 years). Are these rectifiers on their last legs and bound to fail literally any day now, or will they go five, ten more years? Aside from the usual symptom of shorted seleniums (when the part emits a toxic gas that smells of rotten eggs), are there any other signs (such as decreased sensitivity, distortion of the sound, etc.) that the rectifier is about to fail?

BTW, what is the world's record (if there is one) for longevity of seleniums? Are the ones in my radios close to setting, tying or breaking that record?

Kind regards,