The manufacturer apparently does not list an expected life span for the tubes they use in their equipment. Check with their service department for more specific answers regarding tube life. Tubes will wear out at different rates depending on their position in the tuner. However, tubes in a tuner tend to last for long, long periods of time. You should replace tubes when you begin to notice a degradation in performance of the tuner or the tubes no longer test as good. Since tubes wear so slowly in a tuner it's possible you'll have to have the tuner cleaned, realigned and tested before the tubes are noticeably bad. Most tubes used in a tuner should be quite easy to source, the only tubes listed on the main pages are 6922's which are quite common and accessible. Place the type of tube (6922) in a search engine to find retailers and approximate cost for such tubes.