Tube life

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.

" Trying new tubes hoping to improve the sound almost always is a total waste of time. Its not that there aren’t differences. Its that the differences are too slight to bother with. "
Guess I've had difference experiences than you with swapping tubes, for me there have been noticeable differences whether signal or power tubes.

@jrod68Here's
@jrod68H@jrod68Ha@jrod68H@jrod68H link to Brent Jessee's website (I'm only a customer) but he provides listening impressions to various brands within the specific tube type - might be helpful to you

Thanks for the comments (so far).  I can't tell you how much I'm enjoying my McIntosh MA252 with the Maggie 1.7s. 

Darn millercarbon , you can't say stuff I agree with. It is not fair:)

Likely gradual unless a tube starts going bad. You will know when that happens. Most hybrid amps have tubes  in the input (which last a long time) and solid state outputs. Check out Brent Jesse or Upscale Audio for tube options. I would wait a while before jumping into other tubes, so you can establish a baseline that would make it easier to identify the characteristics of  the other tubes.