Vacuum Tube Break-in

In my experience, it can vary greatly from a minimal discernible effect during the first 50 hours to multiple drastic qualitative changes over the course of the first hour or two of usage.

But tubes get hot and metals change when they glow.  I do believe tube burnin is significant...it isn't always positive.  

Agree with @carlsbad and I'll add that the circuit and the entire chain may determine if you hear these changes. I use all NOS or "tests as NOS" and do hear changes in tube sonics, I trust my ears. 

I did try new Linlai 6SN7's which have a 200 hr. burnin (according to manufacturer). I did hear many changes from bad to worse to better. After the 200 hr. mark, I didn't like their sonics even though they had improved. 

     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."

IMO everything has a break-in period but results heard vary depending on your listening skills and the component.