Here are just a few sentences (verbatim), from a letter that Brent Jesse includes, when you receive tubes purchased from him:
"New tubes need a break-in period before they can exhibit their true sonic character when used in audio circuits. Preamp tubes especially benefit from a good break-in. This period may vary widely, but NOS vintage tubes usually need at least 48 hours of use, sometimes up to 100 hours. New current production tubes need 24-48 hours typically. Good break-in can be accomplished by either leaving your tube unit turned on for 2 days in a no signal condition, or you can just enjoy the tubes for several hours each evening and they will be broken in after several weeks."
interesting: https://www.youtube.com/watch?reload=9&v=jwmDf5bSRMQ