Huh?
The rejuvenator can be used for as long as it still powers on..
What I think your referring to is how many times you can rejuvenate a tube.. From what I've read and heard, you can rejuvenate it a few times.. But each time you do it, your lessening the life of the tube.. Which is a good and bad thing.. If the tube needed a rejuve, then you've cleaned it up so it can be used.. Without it, then you prolly would toss the tube anyway..
Rejuvinating a tube is something you do as a last ditch effort.. I usually use a tube rejuve to test tubes and see if guns are toast, look for shorts, and see how much life is left in the tube.. Or that's what I do.. Heh