When to replace preamp tubes?

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. 

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.