The short answer is, as often as necessary.
The long answer is based on the students level of attainment. We recommend that a beginning student practice at a MINIMUM the equivalent of the length of their lesson during the week. So, if the student is taking a 45 minute lesson, they should practice 15 minutes, 3 times a week. Again, this is strictly a minimum, and only suggested for students who have been playing less than six months. 5 times per week is the norm.
As the student progresses, practice time will increase to about 30 minutes by the end of the first year. Intermediate students will typically practice 30-45 minutes 5 days a week. Advanced students may practice upwards of an hour a day.
Your teacher will work out a practice schedule with you to best serve the student's needs. As a general pointer, the practice schedule should be written down in the same calendar as sports practices, ballet classes, doctor's appointments etc. This is important in terms of value-building for the student! If they see that the parents and families deem practicing important, they will too. If it constantly gets "bumped" in favor of other "more important" activities, you will quickly see a downward spiral!! The student will see it as vital if YOU do. It should not be necessary to nag your child to practice if you set the expectation at the outset and stick to it for 21 days. By then the new habit will have begun to form.
Your teacher has many other strategies to help with practice issues should they arise.