I agree with all the recommendations above.
I will also add that it depends on the value of the tubes that are in your Crack.
For casual listening sessions you throw in some $10-$30-ish tubes, you really don't care too much about how long these tubes will last. Most NOS tubes live in the 5,000 hours lifespan ballpark (some are rated for 10K hours), that will last many years, so no concerns there. But say if you throw in some $100+ tubes for serious listening sessions, you'd be concerned with too many ON/OFF switching times.
As as PJ, if I know I will come back for listen within an hour, I will just leave the Crack on. Otherwise, I'll turn it off to save some wear and tear on the tubes.
Also, I don't wait 20mins for the Crack to reach its full potential, actually I normally start to listen to it a few mins after it's turned on. The Crack already sounds wonderful after 3 mins (the difference from its full potential is hardly discernible).