My PrimaLuna amp won't allow me to switch between triode and ultra-linear until the tubes are sufficiently warmed, and my Vincent hybrid preamp has a flashing red LED in the volume knob that goes out when it's ready to play. If those time spans are good enough for the much more knowledgeable people who designed the equipment, I trust their judgement.
Tube Preamps Should Have This Feature
So it's late or you just want some casual listening for a short time. "Should I fire up my tube pre for 30 minutes?". My answer is "No". Why not let me flip a switch (or push in the volume control) to put my pre into "bypass" mode? Bypass would, effectively, convert your pre into a passive device, keeping the tubes dark.
This sounds useful to me.