When parents ask this question it is often at one of the following points in the evolution of the teaching cycle -
The answers vary from teacher to teacher. If your child is lucky enough to have a teacher who works in a pupil centred way then practice time will be a grey area which can be adjusted to fit the circumstances at the time.
The pupils who continue for the longest and continue to want to play the instrument into their old age are most likely to have had a practice relationship which is led by the teacher and where the parent is only asked to intervene when the teacher thinks it is necessary.
Unfortunately many parents (and they of course are the ones paying the bills) take the view that they need to monitor practising from day to day - often without having the expertise to assess the real value of the work done. The result can be unnecessary conflict and a rapid decline in interest in playing the instrument.
If your child has a teacher who works in a self centred way (and many of them do) then a fixed regime of practice times will be laid down from day one. Here the real aim of the teacher is to progress their career or the size of their bank balance and the success of the pupil is seen in that light.
A good pupil matters to them because it increases their sense of personal importance rather than because it widens a pupil's circle of achievement which may, of course, cover many different aspects.
Most teachers will fit in somewhere between totally pupil centred and totally self centred and will advise you from time to time on the amount of practice needed and whether he or she thinks that it is being achieved. If they don't do, ask them.
The most important role you can take as a parent is to provide constant encouragement - (learning an orchestral stringed instrument is a very difficult process) by identifying your child's successes and improvements and praising them.