Leaving tube amps on.

Where do most of the world’s KT-150 tubes come from? And where did all the semi-reasonably priced (if compared to alternative) KT-150 tubes come from?

One large altercation later and those tubes aren’t being imported anymore. 
With Russia doing its thing and China redirecting much of its attention to newer tech development / fabrication, the world supply of current production power tubes is quite a bit lower than it was a few years ago.

KT-150 now = NOS unless perhaps you’re buying the costly version produced outside Russia.

I’ve zero interest in anyone making this political - please do not.

But so many of the power tubes that are unhealthy to manufacture come from two nations that are not up to makin’ em like they used to; that probably shouldn’t be something out-of-sight/out-of-mind for the audiophile who likes using tubes. Turn power tubes off when not in use. 

I have a Raven Audio Osprey with NOS tubes and have asked myself this same question many times. The simple solution that I've come to is: if I'm running the amp, I leave it on for at least 1 hour. This is easy to accomplish because most albums are around 45 mins and the amp really needs 15-20 mins of warm up before it's sounding its best. So, 15 mins before I'm ready to put on a record, I turn everything on and let the circuits settle-in. Then, whether I'm there for 1 record or 10, I haven't short cycled the amp. 

Good Luck!

On my Linear Tube Amplifers and pre amp I try to turn them on a few hours before critical listening and turn off after I’m done for the evening.  The LTA circuitry is designed to maximize tube life so I don’t really worry much about it.  

If I’m stepping away for an hour or two, I leave it on.
Longer than that, it goes off.
The room is so well insulated that leaving them on when not in use heats up the room.