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?
djf1
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.