When to replace preamp tubes?

Each piece of gear runs tubes differently in terms of both current and plate voltage, so there is no general rule that a tube should be replaced in X amount of hours, though the manufacturer of your gear may provide useful guidlines on the specific kit that you own.

I keep several sets of backup tubes. If I suspect the sound is slipping I just sub in another set of tubes. If the sound does not materially change then the first set is still fine.

A tube tester can help you look at certain things, such as matching between elements in dual triodes like the 12AU and 12AX tubes, and can show you if mutual conductance has been compromised, but most don’t run the tubes at the appropriate plate voltage and current so a simple audio test will often yield more information.

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.