What is the required piano practice duration ?
This depends on you and your goal exclusively. For the most part, your piano improvement depends on how much effort you put in. In an ideal world you could be expected to put in hours of work every day. In reality, a minimal exercise should consist of 15 minutes of practice several days a week if the student is attending once a week. It would be more efficient than practicing for longer durations, but only once a week. Repetition is scientifically proven to be the best approach to learning a new thing. It is not much different from the way a runner would practice; he/she would not run for a whole day and rest the rest of the week, he/she would practice regularly. Therefore regular and short duration exercises would be advisable to improve piano playing.