This is a verbatim quote, from a letter Brent Jessee sends, whenever one buys valves from him:
"CARE AND FEEDING OF NEW TUBES: BREAK-IN
New tubes need a break-in period before they can fully 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 in a no-signal condition for 2 days, or you can just enjoy the tubes for several hours each evening and they will be broken in after several weeks.
New tubes, especially NOS vintage tubes, may sound a bit edgy at first, but after the break-in period will mellow out and sound wonderful."