The characteristics of the particular tube matters. In my experience, for example, 6922s wear out when left on continuously but good NOS 12AX7 tubes don't. Unless you can find a specific reference to you tube type I would suggest turning it off based on a 6+ hour cyle time. You should also listen carefully to the warm up characteristics. Some tube gear stabilizes in an hour or two and some sounds terrible for the first 4-6 hours. This might not matter if during the day you are casually listening and doing your serious litening in the evening since the tube would have had the whole day to warm up.
Strategies for preserving tube life
I have a First Sound tube line stage with expensive (and glorious sounding ) Siemens Gold Pin 7308 tubes. I want them to last. My question is, what are the tradeoffs between wearing out the tubes by leaving the pre-amp on vs. stressing them out by powering up and down too often. So, for example, I power down when I leave town for two days. Should I also power down when I leave the house for 4 hours? Or when I go to bed at night? -Dan