This depends on various factors; age, experience and, eventually, aspirations. For a beginner, 10 – 20 minutes may be enough. A child should not be pushed into further practising when he/she is becoming tired. Fatigue can lead to poor posture and a bad technique. An older student with high expectations may choose to practise for several hours. For the rest: something between these extremes, dependent on willingness and other obligations. Remember that how often is more important than for how long.