I would be very cautious about measuring "achievement" with numbers. Music study is a whole lot more than measuring the number of pieces one can play. However, for parents the most tangible mark of achievement is the final result: a learned piece. The problem with this is that the majority of progress, learning, and growth does not come in performance: it comes in practice. That is, the work that is done behind the curtains, the work one does in private practice, the invisible work that takes place in one's brain through mental practice, the temporal growth that occurs in one piece over time as one studies other music. To the onlooker much of this work is invisible, but to the student it is blood and sweat.
As your daughter is approaching the advanced stages of study, know that pieces get longer, more complex, and thus take more time to absorb. Many of the core skills of musicianship only present themselves in these early advanced stages of development. Everything that was learned before was in preparation to learn these techniques. It may seem that in the earlier years of study, your daughter tackled many pieces per lesson, and in comparison to the present level of music is not taking on as much. This is more of an illusion, as those early pieces were meant to introduce the student to musical concepts in easily digestible portions. Now that she has a good amount of knowledge backing her, she is able to work on music that is lengthier and more complex.
As students progress to the college level of study, it is very common that a single piece's performance length is longer than that of the actual lesson! So it is not uncommon for such advanced student to take 6 months to a year on that piece alone.
During my preparations for uni auditions, my teacher requested that I increase my lesson time from 1 hr per week to 1.5 hrs twice a week. It was very obvious that the amount of material, could not possibly be covered in a single hour lesson.