When you are a working musician time is important. I estimate I play around 60 different pieces of music each week. I play for five choirs, for four weekend worship services, with two bands, and in the last two weeks I had two different concert programs to play. There is never enough time to practice everything, let alone practice it as thoroughly as you'd want to.
That's where being able to figure out just how and how much to work on each piece becomes not only a useful skill, a point of pride, but a necessity, a survival skill.
A few thoughts I've collected about how to do that effectively:
I usually gravitate right to the most difficult part of each piece and start to work on it first. My theory is that time--the total amount of time in days and hours between now and the time that I will be playing this in public--is the single most important ingredient for playing the piece well, because the longer I have to become familiar with a piece the more time I can practice not only consciously, but also my brain can work on it in my sleep or while I am doing the dishes, etc. I want to get the maximum distance between myself and the performance. This means that for me Monday is the single most important day for preparing for next week's church service. Most likely I've already been working on the offertory for this week, but if I haven't, I want to hit it hard on Monday so I can gradually work less hard the closer one gets to the deadline. And I want to know immediately if I can handle the hard part or if I need to find something else. I usually have a plan B on hand if I am choosing the repertoire myself, but often I do not get to make that call. In any case, I know the piece will begin to feel comfortable and easy once I have spent a certain number of days on it (I've gotten very good at being able to tell in advance how long it will take me to learn something) and I want to get to that stage as soon as I can, polishing and perfecting, but knowing that if I run out of time I will still manage to get by decently.
As my sight reading skills have improved over the years I can get away with very little or no practice on many of the pieces I play for groups each week. That's important, because there is no chance I would get anywhere splitting my time between over a dozen pieces a day, much less around 60. Some of those pieces are repetitive things, like the church doxology, which I have memorized and can play in my sleep, as well as most of the hymns in the hymnbook. I've learned what I can leave alone, both in terms of whole pieces, and parts of pieces. I also learn more quickly, and often, during rehearsals, I can actually be improving my accompaniment part while simultaneously helping the tenors.
How did I learn to learn fast? By having to do it. Swimming in the deep end is a learned skill. There is no time to worry about it, no time to complain or to stress out. You just concentrate. Hard.
Sizing up difficulties and priorities occurs both on a macro and a micro level. Within a single piece of music there are always notes that are very important and notes that aren't. If I can't figure out which note that double sharp belongs to in the middle of beat four while playing thick chords at a hundred miles an hour I can leave that note out, play the rest of the chord around it and keep going. When you are playing with others they don't have time for you to figure out what you are doing. Many notes double the singers, some double other notes in the accompaniment, most simply fill in harmonies. But some are necessary as rhythmic foundations for the other parts to bounce off of. Or they carry the melody. Or some other bit of interesting musical information without which the texture is incomplete, or at least a lot less interesting. It would take a series of blogs to explain how I tell, on the fly, what notes I can safely leave out if I am getting my fingers twisted or if I need to be helping the altos and can't get all of the accompaniment notes squeezed in at the same time. In sum, I listen. Listen. Listen.
Being able to improvise and compose helps enormously because if I can figure out what is going on I can paraphrase it in a pinch. Being able to read a full score helps too because when in rehearsal with a choir I almost never play the same notes--sometimes I am helping the full choir, or part of the choir, or playing the accompaniment as it is with the occasional important notes for the choir to latch on to, or I can't get the page turned fast enough so I am making playing both staves with one hand (happens occasionally--involves a lot of jumping around!). The result of constantly changing is you learn to do it as a matter of course. It causes one to be extremely flexible because you can't very well complain about not having done it that way before if you never do it the same way twice to begin with!