I posted this earlier today on Gearslutz, but that thread eventually spun out of control, so I saved my post to an island populated by adults.
In my experience, the most relevant factor when using tube equipment is stability of performance: after what length of time will the equipment’s performance be stable enough that no further gain or eq-adjustments are necessary?
This is a relevant consideration, so that takes, edits and punch-ins are predictable, seamless, and don’t vary audibly from one to the next.
In that regard, it has been my experience that the longer you wait, the more stable the performance of tube equipment becomes. The only downside: increased wear, which can be financially prohibitive when replacing high quality specialty tubes.
A reasonable compromise must therefore be found between relative performance stability and reasonably low tube wear.
Here is my data for that sweet spot, from 30 years of experience:
AC701: 15 minutes
VF14: 15-45 minutes, depending on condition of tube
EF86: 10 minutes
6 and 12 Volt Nuvistors: 15 minutes
Miniature preamp tubes (12 Axx/ECC xx, etc.): 20 minutes
Another important question - how will on/off cycling affect tube life - is very hard to generalize.
Many well-designed power supplies for tube equipment ramp up gently and slowly, minimizing wear damage, while other mic supplies or supply sections of mic preamps are primitive, affording minimal ramp-up periods which can decimate tube life.
Here, again, a reasonable compromise between stable performance and tube wear would suggest to turn off tube equipment overnight, with more expensive tubes shut down even for a lengthy lunch break (2hrs+).