Most commercial and military tubes were designed with a 10,000 hour operating life. This life expectancy can be shortened by primarily two activities: 1- frequent power cycling, On/Off [if you are planning a day of listening and want a few hour break, go take your break, but leave the equipment on]; 2: High Bias/hard use- if the B+, or Bias is at or above the design threshold, then the tube(s) will be driven 'harder' which will cause the plate to off-gas, which in turn will eat up the getter, and in short shorten the tube life. There was an amp in the late '80's/early '90s that used 8417 power tubes at Very high B+, and the tubes would only last about 3,000 hours. I have both AI pre-amp (M3) and MR power amp (RM-9MK-1) and have re-tubed each once - after ~10 years of use. The getters were pretty transparent - more like ghosts, and the 9-pins were getting noisy. They (the tubes) are designed to be replaced, and a prudent owner of tube equipment will have spares...
Happy listening
Happy listening