Music is about the only discipline in which simply putting in time is seen as a virtue.
"I practice my instrument an hour a day," is something that you will hear students say with pride.
(Or, "My Mom makes me practice thirty minutes a day.
") Teachers frequently require a certain amount of time per day on the instrument.
When I was first starting out on violin I brought home a paper each week which required my mother to write down how many minutes I had practiced every single day.
I brought the paper to my lesson each week, and a part of my grade was simply the number of minutes that I had practiced.
The results of those minutes practiced often seemed secondary to the number.
  Large numbers of minutes spent in simple, rote repetition is often held up as the standard way to learn an instrument or a piece of music.
It is my contention that this is wrong.
Now before you start shouting that I don't know what I am talking about, let me quash the idea that I am saying that a student should not spend a reasonable amount of time practicing the instrument.
What I am saying is that our focus should not be on the clock or on the number of repetitions played, it needs to be somewhere else.
  Teachers and student musicians should change their focus from how long I practice to how much progress is made during practice.
A two hour practice in which no real progress is made is not nearly as effective as a thirty minute practice with a discernible improvement made in a passage or on a technique.
Admittedly there will be times when the focus is simply on repetition and doing something correctly.
In that instance repetition of a technique done well is a great goal.
But playing a scale twenty times with the same note played badly each time is actually counter-productive.
  Let's focus on an improving what we are doing.
Let's focus on playing the passage or performing the technique, if not perfectly, at least better than we were doing it before the practice session.
  How do we accomplish this?   When practicing a technique, I believe that the key is speed.
Actually, I believe that the key is a lack of speed.
We start almost agonizingly slow .
.
.
slow enough that we are able to perform the technique correctly.
Repeat the technique at this speed until it becomes natural and at least relatively comfortable.
Then we speed up just a little bit .
.
.
just far enough that we are on the verge of failing.
When we have determined where that point is we work at that speed until we have mastered it.
  This is an instance in which a metronome is a great tool.
The metronome helps us maintain the proper speed for our practice.
It is also a great measure of progress.
At the end of a practice, if I can say that I moved from playing at thirty beats per minute to forty beats per minute I have made discernible progress.
  If I am just starting with a piece of music I use a method similar to that which I just discussed with technique.
I start very, very slowly with a passage which I can play.
I repeat that passage at a speed at which I am comfortable until I can play it reasonably well.
  Once I have established a starting point with the piece, I use a practice technique I think of as, "AND .
.
.
?" Here,  I use the part that I can play well as a starting point and I add to it.
"Okay, I'll play this part AND .
.
.
?" and then I add a phrase, or a measure, or maybe even a single note.
Perhaps I will simply try to improve a portion of the selection I already feel comfortable with to make it even better.
I will start as slowly as I need to make it right and then I speed up.
At the end of practice, if I can say that I started with the ability to play twenty measures correctly, and now I can play twenty five correctly, that is discernible progress.
  If I am really practicing instead of just spending time, I almost never end a practice playing something the same way that it was when I started practice.
If I can make substantial progress in forty-five minutes rather than an hour, that's great! I may actually stop at the end of forty-five minutes.
Or I may go on to something that I want to learn for myself, rather than at the behest of a teacher.
Or I may challenge myself to stay with the piece for that last fifteen minutes and really polish it or add and extra three measures! I always end practice with a run-through of the piece as well as I can play it, then reward myself by playing something that I can play well and that I truly enjoy.
That way I walk away from the practice session feeling happy about the instrument, myself, and my teacher!   In summation, focus more on progress than on minutes!