When to replace preamp tubes?

Preamp tubes rarely blow.  They just fade away...
If they lose dynamics or get microphonic it maybe time.  A tube tester is a must or find a local shop with a tester.

It depends on the preamp, some of them are tougher on tubes than others so you really can’t get a definitive answer. You can install new tubes from the same date batch for each channel and one will give it up after 6 months while the other can stay strong for five years. Happened to me a lot over 25 years with the tube-eating CAT SL-1

What I do is every two years is substitute a new set and if I don’t hear a difference I put the old ones back. This way I don’t have to stress over whether the tubes have lost something that I can’t hear because of the gradual deterioration nature of tubes. Other than that, when you hear hissing, pops or crackles, obviously then it’s time for a new tube.

Yes, elevick and gs5556, that’s why I posed the question. They rarely blow and if I’m hearing noise that I can isolate to an individual tube, I’ll replace it. But tubes do degrade. Over time the degradation isn’t obvious. I wanted to hear what other people were doing. The prices of NOS tubes are very strong. So I don’t want to replace them if it’s not necessary but I don’t want to sacrifice the sound quality to save a few bucks either.

What Viridian and some others said.  Tubes usually just fade away.  You may notice a loss of gain, first of all, with tubes in a phono stage. Sometimes tubes also become noisy, but if no catastrophic event occurs, they just lose their mojo very slowly and gradually.  I think listening test is better than testing in a tube tester for determining when to change tubes.  Trust your ears first of all.  Some tubes can look marginal in a tester and actually sound fine.  This is because 99% of tube testers are incapable of testing tubes at the actual voltages and currents to which they are subjected in the context of  a working circuit.