Ok so this is the case. In distortion effects that have a tube like a 12ax7 do you need to change them from time to time or is it good forever if you do have to change them please tell me how often.
The voltage in pedals is very low compared to amps so if a tube lasts 1-3 years in the amp, it will last even more in the pedal. Depends on how much you play actually.
pre-amp tubes (like the 12ax7) also last much longer than power tubes so i say it should be good for at least 5 years.
the preamp tube in my tube driver is over 20 years old and it's still singing
IDK much about tubes, but wouldnt they wear down in a similar way to an acoustig guitars strings?
Gradually, and you usually dont notice the change until you change it?
The voltage in pedals is very low compared to amps
This is untrue for most pedals. There are a few that starve the plates, but almost all tube pedals run at proper preamp voltages.