The life of a tube is greatly dependent on the type of tube, the operating conditions of the tube, and even the specific manufacturer of the tube.
Power output tubes tend to have shorter lives than linestage tubes or the input and driver tubes of an amplifier. Even among output tubes, the ones that are called upon to output high wattage tend to have short lives. A lot of manufacturers, in order to claim high output, create operating conditions that mean shorter life than if they backed off a bit on those operating conditions that shorten life.
While the smaller signal input and driver tubes tend to have a longer life, these too vary greatly in how hard a particular amp drives the tube.
Given that there are so many variable involved, there is no simple formula, such as change tubes after this or that time interval. Both my amp and linestage drive my tubes so gently that I have not had one tube go bad in over 15 years of operation. Even at the start of my use, the tubes that were put in were used tubes of unknown prior usage. Most of these tubes were probably well used before I got them. But, all my tubes are run so gently, they are expected to last a very long time. This is a VERY good thing because they are all expensive. My output tubes are 349s, the driver tubes are 348s; my linestage tubes are 310s.