There are many implications to this general reference. Typically, when I am asked about practice, it is in regards to how often and how long.
How often I can answer definitively. Consistency is one of the most important factors of learning an instrument. Unlike school work, you cannot make up for missed practice by compounding it on other days. A little each day is far more effective than a lot every 2-3 days. It is also important for an occasional day of rest/recovery for the mind and body. That being said, my expectation is that students practice 6 days per week.
How much time per practice is a bit harder to answer. This is determined by the learning personality of the student, which is a very individual parameter. I have found there are generally two main types of learners;
- Goal setters determine what they are going to accomplish during their practice, and put in the time to get the goals done. Their practice time tends to very slightly from day to day. With experience, they learn what goals they can set to fit within their practice capacity, (mental, physical, and literal time).
- Organizers like their practice specific and outlined, either by time allotment or repetitions. Organizers tend to practice for exact time every day.
Both methods are effective. The important thing is to find what works best for the success of student.