I do feel that it is more important to complete/pass off assignments rather than practice a certain number of minutes. However, I still think it is helpful for students to keep track of minutes for various reasons, particularly so I have an idea how long they are actually practicing, and also so they don't just play through their pieces once and think they are done
(more about teaching students to practice later!)
I have my students make their own
practice goal, with my help (for example, a goal could be to practice for 30 minutes a day for 5 days out of the week). That way they have a little bit more ownership over their practicing than if I just told them they had to practice a certain amount. However, I also make sure they have a clear idea of what the
assignments are for the week, and I stress passing those off. I use a
point system, where they earn "points" for doing certain things - 2 points for reaching their weekly practice goal, 1 point for completing an assignment or passing off a piece, 1 point for performing a piece, etc.
Usually a good amount of practice, I have found, is about 30 minutes for young students (although for very young ones with short attention spans and very short pieces, 15 to 20 minutes might be fine), 45 minutes when they are more intermediate with longer/more challenging repertoire, and 60 minutes once they get more advanced. However these are just guidelines, and I think it totally depends on the student, their attention span and their assigned repertoire.
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