A question I suspect reveals a lot of ignorance...

Sounds like a good question to me. And while I've used tube equipment for over 35 years, I admit to not knowing either. What I am reasonably sure of is that for a typical Class A/B circuit, how loud you play will not have any practical impact on the life of the tubes. The impacts of proper biasing and the designer's decision on how hard to drive the tube within the circuit will overwhelm any impact we can have as users in just playing music through them.
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I've heard whenever a tube is on the clock is ticking. That's why I've heard that if you know you're not going to be listening for the next couple of days to power down tubes. I take it one step further with the modern tube gear I own & power down tubed equipment after each usage & allow to warm up for one-half to one hour before listening. I have not noticed excessive tube wear.

But I must be honest, I don't know if listening volumes affects tube life. I suspect it doesn't, practically speaking. I usually listen at 85dB.

I believe that when a tube amp is at idle, it draws full rated power/current. Increasing volume doesn't draw more than its rated power...however, it can drive the amp to clipping, which can be detrimental to the life of the tubes..especially tubes that are worn down, and don't hold their bias.