If I have very little time, and pieces to work on, I might have to sacrifice working on etudes or scales and find what needs most work in the pieces, which hopefully is good for my broader technique as well.
You have to build your own system of triage based on what is lagging in your technique on any given day, and you have to be really judicious to not "overpractice" any given thing so that you can really make good use of that narrow sliver of time.
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It's about choices and working with a purpose. If three-octave scales have never really moved the needle for you, then for heaven's sake don't spend a third of your 30 minutes on them. You want things that will really work your hands and develop your ear. If you have a competition coming up then you might have to consider if you can jettison anything else that is putting so much demand on your time. Finally, you probably want to consider some of the "exercises" that people do WITHOUT their violins, like thinking through a difficult passage while you are riding the subway to your $450,000-per-year lawyer job in Midtown.
I surprise myself by recommending a "self-help book" but THIS BOOK is outstanding. I mentioned it to my daughter who studies at Oberlin and she said that all her professors have that book in their studios.