Average longevity of Nixie tubes varied from about 5,000 hours for the earliest types, to as high as 200,000 hours or more for some of the last types to be introduced. There is no formal definition as to what constitutes "end of life" for Nixies, mechanical failure excepted. Some sources[1] suggest that incomplete glow coverage of a glyph ("cathode poisoning") or appearance of glow elsewhere in the tube would not be acceptable.