I think it will depend entirely on the individual. Some folks who are really good at VO2 Max or Anaerobic efforts may regularly be a 6 or a 7 in that zone but their FTP won’t change at all. And some folks who are really good at sweet spot and threshold may be able to do workouts in the 8 to 9 range by extending time to exhaustion without functionally increasing FTP. As such, I don’t think there is a one size fits all for the question you’re asking but more thinking critically about your recent performances and if you think it is time to retest.
And in any case, you have to use Plan Builder for AT to work to the fullest extent at the moment, so you’ll be asked to do ramp tests at the beginning of each block just like you would with the static plans.