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Too many teachers say things like "you must practice for 15 minutes everyday" - that is one sure way of making sure you don't practice at all,
Doesn't matter whether you ask them, tell them or don't mention practice at all, some students just don't practice,...but they're quick to whinge the following week that they can't play the piece/study/scale

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because you'll have already thought of all the other interesting things you could be doing in 15 minutes!
Or in the case above, wasted 15 minutes just flicking aimlessly through 200 TV channels.
I've been playing the violin for 21 years. I'm a professional, but it doesn't stop me from practicing. I guess on average I still do around 2 hours practice a day - sometime more, sometimes less, a lot depends on work.
Practice would consist of a brief warm-up and a couple of scales. Work on some technical issues (yes, I still have a lesson from time to time as we can all learn new things or get our bad habits corrected), maybe do a couple of studies, or sections from them that relate to the technical issues, then I'll either move on to one or two repertoire works, or more likely orchestral excerpts, especially any that are being rehearsed for concerts.
Whatever I practice, I concentrate on the bits that I know I'm finding difficult. It's a common fault to waffle through things you find easy and to keep playing those bits, and all the while make yourself believe you've practiced. But real practice is all about working on and learning the more complicated sections of a work. Rallentando shouldn't mean, 'slowing down because I can't play this bit very well'.