Best Way To Maximize Preamp Tube Life?

I would love to learn how to best maximize tube life. Tubes have a limited lifespan, of course. So when you're not listening for a time, is it best to shut everything off to preserve the "hours" left on the tube's life? OR does the act of powering off/on itself shorten tube length as well? If so, by how much? Something like "powering off/on costs 3 hrs of tube life, so taking a music break of less than 3 hours, better to just leave it powered on." Or 1 hr, or 10 minutes, 6 hours, etc? Where is the tradeoff point?

In my system FYI, I am running a Don Sachs preamp with 4 6SN7s and 1 6BY5 rectifier.  Don says the preamp is only running the tubes at 40% of their rating. I would greatly appreciate some input from people with tube knowledge. Thanks in advance!