I teach a lot of different kinds of students, from college students hoping to pursue a career in music, to amateur adult musicians who have careers but love to play as a hobby, to my own children who take lessons and ask me about their own practice routines.  Some of them tell me their teachers are very specific about how long to practice each day (and with young kids I can see why they do that).  With my college students I see that there are a lot of competing demands on their time, and sometimes they don't know how to make the best use of limited time.  Here's a great article I found on the subject of practice time.
 

http://www.bulletproofmusician.com/how-many-hours-a-day-should-you-practice/