In Session A you are working with a specific anxiety that is generally precipitated by a specific circumstance. In Session B you are working with more amorphous anxieties, which may be more pervasive and don't seem to have a clear or precise cause.
There's no cookie cutter answer to when it's time to move on, but generally there will be a loosening of the intensity of the anxiety you are working with and a sense that it moves through you rather than grabbing hold of you. The energy and power that was bound up in the fearful, anxious thoughts are then available for more constructive and creative outlooks and actions.
All the best!