Oh you could check the bias once in a while. But mostly we set it when they are new, and pretty much let it be.
If the tubes are showing signs of failing - pops and crackles, flashing, or other things - then get rid of them NOW before they damage something.
Otherwise, you know when to change them the same way you know when to change strings - when the tone has gone away. They will still function long after their tone has gone lifeless.
I'm in Lansing, where are you?