Small signal tube life, (example 12AX7 and 12AT7), for new production tubes is about 4000 hours average. NOS signal tubes from the 1950s, 1960s are said to average up to 10,000 hours of life.
The average tube life depends on how hard the designer of the amp pushes the tube in the circuit design.
New signal tubes as a rule gradually wear out slowly over time.