When a new student begins lessons they or their parents will often ask me “how much should I practice?” This is a very good question. My stock answer is “20 min daily minimum”, ½ hour daily would be good and if you are having fun practice as much as you can.” This advice is meant for beginners. Other people may have different goals that would require more practice. The important thing is to practice regularly. Try to do some practice every day, even if you only have 10 minutes. 20 min a day is better than 60 min every 3 days. Your mind and your muscles will retain the training better if repeated frequently.
But just as important, maybe more important, than how much you practice is HOW you practice. I’ve noticed that most beginners tend to practice this way: They play through the piece from the beginning. They stumble in the difficult sections. Then they either play to the end and re-start at the beginning or go right back to the beginning from their “stumble point”. Either way they play it again pretty much the same. They keep repeating this process. Some of them will eventually get the piece perfect. But most will get really good at the easy sections and never master the difficult passages.
A better approach would be to note the difficult sections, even marking them with a pencil. Then work on just the difficult passage. Ask yourself why you stumble. For instance, maybe it is difficult to move your third finger rapidly from string 4 to string 2. If so, repeat that movement until it becomes easier. You may have to “do it in slow motion” at first and gradually speed up until you have it at the song tempo. Go through the difficult sections one at a time, they may be only a bar or two long, and “fix” each of them in this way. Then take another try at playing the piece from beginning to end. Many songs or exercises have easy sections or may be mostly easy. Don’t waste your valuable practice time on repeating sections that you can easily sight-read without error. Work instead on developing the skills necessary to play the difficult sections.
Teachers are not perfect and sometimes they give you too many assignments. You may be better to put one piece on hold and perfect the one piece you have time for. Your teacher will be more impressed with your progress if you played one piece beautifully than if you played two pieces poorly.
Lastly, before you end your practice section, play something just for the sheer enjoyment of it. It could be rocking out along with your favorite track, or playing something easy that you just like the sound of. Practice time should be enjoyable. If it is something you dread, you will find ways to avoid doing it. If you are feeling this, talk with your teacher. You may find them more understanding than you thought. They have probably been through it themselves at some point and they may have some good advice.