Hi attmci,
When I started tube rolling some ten years ago I was very skeptical about so called burn in. However, over time I have been convinced that it is true, both about electronics in general and tubes in particular.
Different tubes require different burn in time - perhaps the longest takes with tubes that do not get hot in use such as the EL3N, EL11, EL32 etc with upwards of 400 hours for some.
Many tubes seem to stabilize after 30-50 hours in my experience.
To my surprise, solid state equipment may need time as well to sound their best. I just got a Jotenheim from the St company (don't worry, I haven't defected to the ss camp - somebody gave it to me), and it needs several days of staying on until it sounds it's best. Maybe it is something about stabilizing the operating temperature - I just don't know. All I know is that it sounds better after being on for a while; something that has been corroborated by others.
I aware that there are people who claim that the tubes sound the way they sound right away and that the sound does not change, but this has not been my personal experience.
Come to think about it, I am questioning people that roll new tubes in rapid succession and then instantly proclaim which one sounds the best. As far as I am concerned, it takes quite a long time to formulate an opinion, especially if the tubes sound similar to each other.
I have even read about people who claim that all tubes sound the same which just proves that we hear things differently. IMHO there is plenty of room for all opinions - the main thing is to enjoy the music......