The amount of practice time depends on the child's musical goals.
In my area, there are teachers who teach music to students whose goal is to learn a particular instrument because of interest, explore their musical talents and have fun with it. These teachers allow the students to set their own practice times and provide guidelines on how a practice session would look like.
There are some other teachers who only take on "serious music students" - these teachers move the students to 1 hour classes very early (sometimes 2-3 times a week) and then require a certain amount of practice time every single day. These students are bound for competitions (at the very high levels) or to pre collegiate music programs at prestigious music conservatories. They are also required to take Musicianship examinations annually, learn music theory, sight reading, ear training etc and perform in recitals as assigned by the teachers. These kids practice from 3-6 hours a day and though they enjoy it a lot, it is really hard work.
Where your child lies in this spectrum is something only a parent can know. As a general rule of thumb, when my child started lessons at a young age, the teacher told us to practice for a length of time that was same as the length of the lesson with the teacher (e.g. practice 45 mins a day for a 45 min lesson with the teacher). As my son advanced, the teacher told us to practice for as long as it took to cover the material that was assigned for homework - at that stage, a concept that took the teacher 10 minutes to teach in a private lesson took much longer than 10 minutes of practice per day to master it.
