I take private lessons, but my music teacher doesn't discuss how long and how often I am supposed to practice. I am in middle school, I practice alone, I play the viola, and I am at an intermediate level.
It's usually better to practise regularly (say 5 days a week?) and only as long as you can retain your concentration and focus. That's obviously a very individual thing. Shorter, regular sessions are better than intermittent long ones. You have to work out your own balance. Try to get into a routine - eg. start with scales and arpeggios, then exercises and studies, and then pieces. However, keep it fun! Do pieces that you like. Also, try to find like-minded people to play with - string quartet, youth orchestra, etc. Being a viola player, you should be in great demand! :)
It all depends on how good you want to get. If you want to play really well, you have to practice a lot. If you don't care, then you don't need to practice as much.
When you do practice, you need to get the most out of you practice time. You have to make sure you're playing the right exercises in exactly the right way and with full concentration. That often means shorter practice sessions, but regularly.
The usual advice for training prefers frequent but shorter sessions over infrequent long sessions.
It's hard to say exactly what numbers to use. There are psychological factors as play. But, I think a minimum of 20 minutes a day, everyday is a good starting point.
Everyday practice is a goal. You might miss some days. The point is to make it a natural daily activity.
You can practice longer than 20 minutes, but don't make it drudgery. This brings up an important point. It matters what material you practice and how you spend the time.
You want to be thoughtful when practicing. The to find the technical or musical reason for practicing. A common bit of advice is practice what you don't know until you get it then move on to the next challenge.
20 minutes of focused and thoughtful practice is better than a full hour of robotic repetitions.
These are all good answers. @michael-curtis makes the key point regarding thoughtful as to robotic approaches. Unless you are one of the 1% of the 1% of musicians born with what can only be described as the "gift", then you need to do real work in order to attain any level of competence. Jascha Heifetz has been quoted as saying that if he missed 1 day of practice, then he noticed the difference. If he missed 2 days of practice, then his audience noticed.
In practical terms the following are basic and inalienable facts:
- It is easier to learn when you are younger.
- In any field of competence, it takes about 10,000 hours to attain expertise.
- Limit your practice sessions to 45 minutes to 1 hour. Brain fatigue is real and it is genuinely unproductive to push oneself longer. Multiple sessions with a 15 minute break between sessions seems to work best.
- Twitter, Facebook, videos, texting, etc. are nothing more than mindless fads. What would your 70-year old self say about the time spent in these distractions as opposed to learning music?
- Learn what you love.