I went about 4-5 years on the stock tubes in my SVT. It all depends on how much you are playing. Seeing this is you and I know you play a ton you still should easily be able to get a good couple years out of tubes. The thing is you don't need to re tube just based on time. If the tone is on and you aren't noticing a drop in output, the bias is good or something is wrong with the amp you can get away without changing them for years. Working in the store I have seen 70's SVT's come in and still have the original tubes in them working fine. Granted I am sure with a nice new set the amp would kill but they still work. if someone was really thinking of giging out with it I would suggest having them changed. When you re tube just get something good and they will last you a long time. If you are giging out 200 days a year then I could see changing them once a year but other then that just listen to you amp, if it sounds good and there is no real problems with it you'll be fine. You'll know if one goes.

I'm not exactly sure why since I don't understand it, but this is hot: