As with most people with AS, I feel less than good most days. Occasionally I feel much worse than my normal baseline. Those are what I think of as Flare Ups, but the length can be one day to several. It’s just not something doctors teach or train us to identify, so I guess its kind of different but sort of the same for all of us.
In addition, I hate the how do you rate your pain today? question at follow-up appointments. They never seem interested in how good or bad I might have felt on any other day.