I aim to spend about one-third of my practice time on scales and etudes, which typically means 20-30 minutes when I'm on my normal practice routine (currently not doing it because of injury).

The way my practice sessions have been organized, for most of the time I have played, is: start with scales and etudes for 20-30 minutes, work on repertoire for 40-60 minutes, and then if necessary finish with 5-10 minutes of exercises on a single technical issue that I may have encountered while practicing repertoire.

I do not go through the entire set of scales and etudes I'm working on each time I practice, but rotate scales and plan to give each etude a certain amount of time each week. (I may practice an etude every day if I'm trying to improve one particular aspect of my playing.)

Similarly, I do not go over all of my pieces in any single session -- in fact, unless I'm just about to audition or perform solo, I don't play through the entirety of any piece more than once every week or two. I focus on shorter segments with the most time devoted to the trickiest passages, and have been known to spend an entire practice session on a single measure of music. If I were to play through everything I'm working on, it would eat up all my practice time without any opportunity to work in detail. Instead I just plan in advance to play through a piece on a certain day.