Another question that is like "how long is a piece of string?". It depends on what you want to use it for. If you want to treat someone for severe anxiety and you only have a limited number of sessions, you might want to achieve as large a change as possible. Even so it would be important to know how scores might vary over time if the condition did not change. That would depend on the test-retest reliability of the scale, in this case over multiple administrations and over a longer period. Some people improve their scores over time due to familiarity with the questions, without any change in their real anxiety levels. In addition, a person developing severe depression even though not being treated, might have a pattern of increasing anxiety scores. Improved scores can happen because people can expect or want positive change even though it does not happen. There are published studies on test-retest reliability for this scale and those estimates can be used to profile raw scores and their range when this might be due to chance. Then you could test your obtained scores around a 95% confidence interval to see if the observed score is significantly different (outside) the range that could be due to chance. So in short, do not assume that the sub-clinical score or the baseline score will be stable, even if the condition does not change.

As Geffrey said, repeat testing of individuals is an unreliable measure of improvement. Patients selected because of particular symptoms will generally have less severe symptoms because of regression to the mean. And on top of that, people tend to rate their own symptoms less severe the second time they complete the questionnaire, even if their true symptom severity has not changed. In clinical trials, these can be accounted for by including a control group. In psychotherapy, the effects mainly work to convince psychotherapists that their therapy caused improvement, when in fact much of the change in scores was due to the aforementioned effects.

The minimal clinically important difference (MCID) for the Beck Anxiety Inventory (BAI) is generally considered to be around 10 points. This threshold indicates a meaningful change in anxiety levels that is perceived as beneficial by patients. However, specific values may vary depending on the population and context, so it's important to consider the particular study or clinical setting.