1) Some pre-amps are 'hard on tubes' - usually the manufacturers will acknowledge this and will recommend using only very rugged tubes (Audible Illusions and Audio Research come to mind. ) Under normal conditions with most tubed pre-amps you should be able to get at least 3000 hours. (You can do a search on line to find out Rogue's reputation on this unit, or call Rogue for the real skinny.) Some a lot more! 10,000 hours is not unheard of.
2) For small tubes 24 hours will reveal their sonic signature. Change thereafter will be nominal until old age sets in.
3) Small tubes in both pre-amps and power amps are influential on the output, the pre-amp perhaps more so because it output is dependent on one tube type, but with a power amp its sound is based on a matching between the power tubes and the small tubes which gives a lot more flexibility in the sound of its output, i.e. you can fine tune it more easily.
4) JJ tubes can be fine if you like their sound and they are low noise. I have used them often without problems. I usually change them out only to change tone. Good life expectancy and they are cheap(!) so you can have a few spares around should you need them. Since you already have them just enjoy and wait for the time you want to explore sound changes using different tubes.
2) For small tubes 24 hours will reveal their sonic signature. Change thereafter will be nominal until old age sets in.
3) Small tubes in both pre-amps and power amps are influential on the output, the pre-amp perhaps more so because it output is dependent on one tube type, but with a power amp its sound is based on a matching between the power tubes and the small tubes which gives a lot more flexibility in the sound of its output, i.e. you can fine tune it more easily.
4) JJ tubes can be fine if you like their sound and they are low noise. I have used them often without problems. I usually change them out only to change tone. Good life expectancy and they are cheap(!) so you can have a few spares around should you need them. Since you already have them just enjoy and wait for the time you want to explore sound changes using different tubes.