Some people will tell you that tubes degrade every time you turn them on and that is actually true, but not in the way that people would normally interpret degradation. The life of a tube is degraded every time it is turned on vice the sound being degraded. In a perfect world, all tubes would die a natural death from cathode depletion. Every time you turn on a tube and you are conducting (heater, cathode, and plate voltage all on), you are emitting electrons from the cathode to the plate and cathodes have a finite life with regards to how many electrons they can emit.
The accepted rule of thumb for output tubes is that they are good for around 2000 hours of “on” time. As output tubes age, you gradually have to compensate by adjusting the bias in order keep the bias set to specification for the amp it’s used in. At some point in time, you will no longer be able to bias the tube correctly and it has to then go on Ebay and sold as NOS (just kidding).
Many small signal tubes are rated for lasting up to 10,000 hours although companies like ARC recommend replacing the small signal tubes in an amplifier on every second output tube replacement which would mean 4000 hours. Lots of small signal tubes go noisy/microphonic before they ever reach their end of life due to cathode depletion.
