If it has been awhile since you last checked your bias, just hook up your meter and check the bias readings. If they need to be adjusted, just adjust them.
The (maybe just "my") ancient quickie rule was 1-10-100 -- check at 1 hour, 10 hours, and 100 hours. 1 hour to catch any initial burn-in drift, 10 hours after they have settled a bit, and at 100 hours they should be in "long-term" mode. I would check every few months after that, which might be 100 - 300 hours -- whenever I thought to do it. Some tubes would drift very gradually then change rapidly at EOL (end of life); others seemed to drift slowly at first then a little faster as they aged until they died.
Just a rough guideline; some will check after every record or CD, at least until they tire of it, and some will check when they replace them and never again. The 1-10-100 rule worked pretty well for me over the years.
The (maybe just "my") ancient quickie rule was 1-10-100 -- check at 1 hour, 10 hours, and 100 hours. 1 hour to catch any initial burn-in drift, 10 hours after they have settled a bit, and at 100 hours they should be in "long-term" mode.(...)
Although I broadly agree with the timing my wording would be different for ARC modern amplifiers - 1 hour for temperature and amplifier stabilization, 10 hours for leaky tubes and 50-100 hours for burn-in. I have measured several sets of 6550C and KT120 during burn-in with my burn-in and matching jig and found sometimes they take about 30 hours before fully stabilizing bias.
If people get tubes already burn-in from ARC they can follow ARC instructions.