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
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?