How long you practice depends on several things including level, goals, and time. If you are a beginner 15 minutes is probably enough time for your daily practice. Intermediate players somwhere between 30 minutes to an hour per day. Advanced players a few hours per day. Professionals likely four hours and up depending. A daily routine is good, but if you have a busy schedule every other day too can work. Have a plan for your practice. A practice journal can be very useful. Write down when you practice, how long, what you practiced, and any questions you may have for your teacher. Remember listening to music can be a part of your practice.