Kijanki
Given the question at hand was not about longevity, and rather, the need to remain on only, I thought to throw that Odyssey tid bit in too.
The Stratos SE amp only takes a couple hours or so of being re-energized for there to be no discernable diffs. I use it now, strickly as an HT amp anyways, so it sits at rest 90% of the time... though it is powered up and supplied by a RSA Haley power filter.
Tubes I use as needed. Another owner of the same preamp as mine said to get better sound, "Leave it on"... I tried that a couple times and found either little or no difference after a few days of being powered up, so it's on only when needed now... about an hour prior to playing some music. A half hour of music and then I'll take a seat and listen.
I do the same thing with the HT rig too... though for about half the time spans, 20 on & 20 more with signal.
The FPJ does not like to be left on at all, so as it's switched at the wall, I'll hit that too just prior to use.
I'll still agree however, if the conditions I posted above can be met, keeping things on will help extend life expectancy and reduce failures. Albeit, the costs of ownership are increased.