As long as you can without discomfort (pain, tension, tingling sensations) in your body and with full mental concentration. What is more important than the length of practice is the frequency and consistency of practice. The more practice sessions you can get in a week, the better. You will also get more results if you practice on a regular schedule. In other words, one 15 minute practice session over 5 days in week will progress you faster than one single 75 minute session the day before your lesson in a given week. Thus, it’s more important to schedule a reasonable amount of time in a sustainable schedule. The point is to build a habit of practicing into your routine over the long run.
