What does the amber light on the AirPods case mean when AirPods are not in the case?
Apple's support article for AirPods doesn't point to an exact percentage but rather if the case can fully charge the AirPods.


  If your AirPods are in your case, then the light shows the charge
  status of your AirPods. If not, the light shows the status of your
  case. Green means charged, and amber means less than one full charge
  remains. If the light flashes white, your AirPods are ready to set up
  with one of your devices. If the light flashes amber, you might need
  to set up your AirPods again.