How Long?

See above. The same constraints apply with regards age, standard, level of concentration etc. It will be different with each student. In any case though, allow for periods of 'fiddling' and games on top of the homework that a teacher sets, either within the practice session, or maybe in a second daily session. For instance, covering the set practice in the morning then leaving the pupil to do whatever takes their fancy in the evening - improvising, games, messing about finding tunes etc. Ideally when new technical habits are being formed then even the fiddling should be mindful, otherwise the good work undertaken in the set time gets undone as old habits take over when concentration is elsewhere. However, paramount is an enthusiasm and curiosity for the music and instrument so it's up to a parents' judgement how much to intervene when a student is absorbed in his or her own world at the piano.