Let's begin by talking about how to take a timed exam. It seems an odd place to start, but bear with me.
When taking an exam, go through the test and read ALL the questions. At the same time, answer any of them that seem easy and obvious.
On your second pass through do the same. You will find that there are many questions that seem so easy and obvious that you'll wonder why you didn't spot them on the first run through.
On your third pass many of the questions that you were unsure of on the first two passes will now seem crystal clear. How did something that was originally so cloudy suddenly become crystal clear?
The answer is that the subconscious mind works on problems in the background while the conscious mind is occupied with other matters.
Properly utilising this amazing characteristic of the human mind allows us to practice more efficiently and economically.
Have you ever sat at your instrument for hours and hours practising and practising and practising the same thing over and over and over and yet in spite of all your hard work you're not getting any better at it and you feel that you're just beating your head against the wall?
In fact, you ARE beating your head against the wall!
The most effective way to practice is in short bursts with relatively long breaks in between practice sessions.
Marathon practice sessions are a very inefficient waste of time. You get much more bang for your buck if you stagger your short sessions.
Back to the burning question of "How long should I practice every day?"
If your goal were to practice 1 hour per day (not an unreasonable goal, but it's often difficult for adults with their heavy schedules to find an hour's worth of free time each day), you would be much better off to do four 15 minute sessions or six 10 minute sessions with breaks of a half hour or more in between.
If your goal is to practice more time, simply add more short sessions into your day.
TAKING BREAKS IS AS IMPORTANT AS THE PRACTICE TIME ITSELF. This "downtime" allows the subconscious mind to "work on the problem in the background". When you come back to it, you will be surprised at how much easier it is when something's allowed to "simmer on the back burner" and digest for a while.