It is true to say that learning to practice effectively is something that takes time to master.
All practice is not created equal. There are good ways of practicing and not so good ways of practicing.
Left to their own devices most students will practice inefficiently. This means that they are not making the most of their practice time and not benefiting from the time they do spend practicing.
A large part of a piano teachers job is to coach students on how to practice effectively. Practicing is a skill that must be developed over an extended period of time. Practicing should be something that is done thoughtfully with the student fully engaged in what they are doing.
I have met many advanced musicians that have still not learned to practice well. The journey that these musicians have taken on the road to musical proficiency has often been difficult and arduous as a result of their self-imposed routine of uneducated practice techniques and it is true to say that by neglecting to learn how to practice properly they have unwittingly placed a ceiling on their own ability to progress to higher levels of performance.
One difference between a good piano teacher and a great piano teacher is that the great teacher will be an expert at helping students master the art of practice. Students must be taught to be self-sufficient in their practice and approach it in a way that continually identifies and solves musical problems. Unfortunately, as a student or parent it is almost impossible to know if you are being guided in the right way regarding the development of a sound practicing approach by your teacher and it is sadly true to say that we often don't realize that the lessons we have been taking have not been very good until it is far too late to do anything about it.
Don't fall into the same trap as so many other parents and students who believe they can achieve their full potential with the cheapest teacher they can find. Poorly done lessons often do more harm than good. To become a pianist is a far more involved process than you may first imagine and expert guidance is essential from the very beginning if you want to give yourself the best possibility of success.
Learning to practice properly is a skill that must be developed over many years and cannot be learned by reading a few online articles about practice. It is something that you have to do and experience to fully appreciate. Having said that there are many good articles that can help give a snapshot overview of the type of work involved in the development of a great musician. For more information please read this excellent article called "How Many Hours a Day Should I Practice" by Dr. Noa Kageyama.