I have a 1973 Fender Super Reverb and I'm still using the original tubes. Granted, I have not used it an hour every day for the last 27 years, but it has seen its fair share of use. Your question is a good one, though. I had the same concern and bought 2 full sets of replacement tubes. I've yet to use them. So if you can afford it buy the replacement tubes and you'll be ready. Let us know when you need to replace them and maybe we can figure out an "average" time to replace them. Hope this helps.