I prefer to ask how often, as frequency makes a big difference in a student’s progress. Piano playing is a difficult skill to develop, and the fewer days of practice, the harder it becomes. When students aren’t able to practice daily, they become frustrated because they are not able to play a piece they could play better before when they were practicing regularly. They will also not advance on to new pieces as quickly so they will become bored with the music. Daily practice, however, will make the most of the lessons and will keep piano playing fun and enjoyable.
For beginner piano students, assignments will take roughly 20 minutes a day to complete. However, if there is a day that they only get 5 minutes, I prefer that over 0 minutes! If a student practices more than 15-20 minutes per day, generally they will progress at a faster pace. As a student progresses, practice time will need to increase, but generally they should expect to practice 45-60 minutes per day.
Practice time can be broken up to have the same if not better effect. So, 15 minutes in the morning and 15 minutes after school works just as well, if not better, than all at once.