When to replace preamp tubes?

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.
Bob

My apologies for a lack of clarity in my last post. Preamp tubes tend to age fairly linearly like a correctly aligned tire. Like tires, there can be catastrophic failure, however it is much more rare than linear wear.

Power tubes are another matter entirely.

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  
"Motorboating"  
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.