I am not sure why they need a day of rest. And why only over time? When you turn a tube on, the current inrush spikes. On some tubes, it may be amps higher than the normal operating point. Shutting it on and turning it off would do more damage if it needed some form of break in. Heating it over numerous hours would do whatever burn off needed to be done without the current inrush stress on the filament. The latter statements assume a fixed voltage heater element circuit. If there is going to be filament failure, it will be on turn on or due to mechanical shock...
I am not sure why they need a day of rest.For the chemical reactions of the getter fully happen until the end of the process.
Inside a valve there is millions of molecules of oxygen and other gases. Who will clean this is the getter to made a vaccuum that have safe condition to the tube operate in hi temp.
Unfortunately the getter remains active only about six months, after this period it is necessary to make a new burnin.
And why only over time?Not sure how many hours from burnin is recommeded for the Elrog 300B,
together w/the tube must came a paper informing this very important info. If the builder dont inform this info it is a very bad sign for the people that own this expensive tube.
When you turn a tube on, the current inrush spikes.The worst thing you can do for a valve is put it into an amplifier without softstart and without two power on switches.
In this adverse working conditions a chinese big triode lasts only 500 hours and a robust Russian GM70 about 1200/1500 hours.
Even a 6C33 used inside a MIG25 jet need softstart, in this case (military use where cost is no object) its recommended 120seconds minimum time.
Unfortunately its only mentioned in 1 data sheet made by Michael Boele:
http://www.jogis-roehrenbude.de/Russian/6C33C/6C33C-B-6S33S-VExtendedDatasheetMB.pdf(Warm-up time to 90% emission 120 s.)
To put a right tube amp in great shape the power off procedure also need ''softstart'' or wouls say ''softpoweroff'', I dont know how implement it unless with a simple Variac as used by expensive Joule-Electra OTLs.