Guitar practice is an important issue and how much you do really depends upon your level, but it is something that you should do every day without fail. I used to practice the guitar for 12 hours a day, but I can achieve the same or better results now in a few hours through structured & planned practice sessions. For a beginner I would suggest a minimum of between 15 to 30 mins. a day, every day. As the pieces you are learning become harder and your workload becomes heavier with the addition of scales, arpeggios, & technical studies, more time will be needed.
Here are some fundamental tips to gaining the most from your practice time:
1/ Set aside a time for practice every day.
2/ If possible, try to schedule your practice session at the same time every day to establish a routine.
3/ Set a goal for each session, e.g. four bars or a line of music.
4/ If studying at school, do your practice before your homework.
5/ Divide your session into sections, e.g. 5 minutes scales & arpeggios, 5 minutes technical work, 15 minutes music.
6/ Do your practice in a quiet place with no distractions.
7/ Stay alert and focus on what you're trying to achieve.
8/ Keep a log book for practice, and make a note of any questions to ask your teacher at the next lesson.
Setting goals for your practice sessions is an excellent way to make progress. Goals give you a game plan and direction, and after a while you start to become very efficient at learning & memorising a piece of music. Even if you fail to achieve your goal for that day, you can rest assured that you've still made good progress and are advancing. If you ask your teacher I'm sure he'd be more than willing to help you arrange your practice schedule.