When to replace preamp tubes?

When do you replace preamp tubes like 12AU7sand 12AX7? Do you wait till they blow or start making noise? It seems I’ve had them in for a long time and are in use everyday. I have replaced them in the past when they become microphonic but should I wait that long, should I just replace them after a certain amount of hours?
Preamp tubes last many thousands of hours. I have antique radios that are over seventy years old and still have the original tubes. Even power amp tubes have a long life span! I've never had a tube that went microphonic. 
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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.
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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.
^ Spot on. Tubes just lose their mojo very slowly. In my preamp new Golden Dragons last for about ten thousand in great working order but after that they just start to fade away. They simply start lose their dynamics and detail so the sound becomes weak and eventually just dull. Once I used for twelve thousand hours and went surprised how anemic the sound actually had been when comparing to a new set of tubes. I have never experienced microphonic nor blows in my preamp.
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In my experience the answer is - much sooner than you think. In the case of my old ARC Ref 2SE phono stage I noticed appreciable noise in one channel at 400 hours and replacing the set of 6H30s was a transformation in sound. I think factors such as on/off cycles probably play a part too, my listening sessions tend to be short so I may be hard on tubes. The only real test is to keep a broken in replacement set on hand and swap them out every several hundred hours or so once you get to say 500 or 1000
This is a great thread.
I am new to tube amp/preamps, and reading these opinions help a lot.
Luckily, Mr. Karsten is around should anything go astray.
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Better NOS tubes last longer or it depends ?
My opinions, partially based on experience from 55+ years of using tubes, building tube electronics (mostly from kits) and tweaking tube electronics.  I'm no expert, but I saw one on TV about 20 ago.

How to tell if a tube needs replacement?

Increased noise in one or both channels
Loss of sharp imaging  / soundstage
Tube fails to glow (burned out heater element - "hard down")
Persistent humming in one channel even when a non-connected source is selected
Tube goes microphonic (tapping on chassis can be heard through speakers)
Increasing distortion or output level in a single channel  
Bright glowing of tubes, particularly accompanied by humming, sometimes LOUD humming (brighter in one tube than others)

Errant tubes can sometimes be identified by switching noisy or microphonic tubes to different channels to see whether the noise / microphonic follows the tube.

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