I'm wondering if you're ever "done" or do you just go practicing them forever?
My father was a French horn player for the New York Phil and later the Pittsburgh Symphony. It was a rare day when he didn’t go down to the basement and play a series of scales and arpeggios, sometimes for extended periods.
I talked to him about it, but years ago so I can’t tell you if the following thoughts are his or mine:
There’s a lot of repetition in music. If you’re getting paid to play a set, you’d better find what you love about that set, or you’ll burn out. I think practicing the fundamentals is a way of setting your mind to the task of getting past the mere rote of practice and finding the infinite depth within. When you play a complex piece, there’s always something to work on right at the surface–a difficult passage, some dynamics, some phrasing. But when you play fundamentals you focus on deeper stuff: pure tone, vibrato, subtle intonation choices, etc.
I'm told that my great-grandmother, a professional pianist, practiced her scales for some hours each day. I don't think that you're "ever done," unless you've decided to quit playing.
As with any sort of athlete, you have to practice to keep your skills sharp, and scales are one of the best exercises to do. You could probably substitute other exercises, but scales are an important part of most music. Practicing scales will make scale passages in music quick to learn.
Liszt was said to practice his exercises while reading a book, as did others. Most of the great musicians spend some hours each day on technical exercises. (Horowitz was something of an exception; he was said to practice no more than a couple of hours a day most of the time.)
Finally, here's an amusing story, very pertinent to the subject, that Itzhak Perlman tells of his first meeting with Heifetz at age 14.
When i was at high school we had one particular violin teacher whose morning ritual was to do 10 mins of scales as soon as he got into his teaching studio. I know that all of the music staff also maintained a habit of regular scales & arpeggios, but Mr. F's habit was most conspicuous. Scales & arpeggios provide a great way to warm up one's instrument & finger/lips/tongue/hands.
So no, you're never "done" with the basic technical exercises.
Famous pianist Vladimir Horowitz did not do it as per one of the interviews he gave. He said that he did not like the mechanical approach. Practicing should be musical and not mechanical. He considered scales as rather mechanical.
Also pianist Andras Schiff says what he does is to play some piece from Bach instead of any sort of finger exercices.
Playing an instrument is a physical skill, just like gymnastics, football, athletics or stage magic. No-one would think that Usain Bolt would stop training after he became world record holder and still expect to win races would they?
I think that it is the same with an instrument. You need to maintain the skill, the dexterity and, in some cases, the breath control as well as keeping the flexibility and hand/foot/breath co-ordination.
Scales are one of many ways to do this but they are a fundamental part of any instrumentalists technique so to stop practising them would be, in my view anyway, a mistake.
I don't think that practising scales is ever a waste of time.
I'll also note that from the angle of an improvising musician, practicing scales and arpeggios moves the mechanics of getting from note to note into your muscle memory, and this makes it much easier to improvise, as you can think "I want to play this" and your fingers/lips/etc. know how to do it without you having to concentrate on the physical details.
You are never "done" practicing scales or arpeggios, but you can make this process more musical.
I still practice scales and I don't plan stopping. However, usually I do not play the standard major/minor scales, but rather random others (frequently artificial self-invented too). Same with arpeggios, rather than using standard major or minor chords I like to use "jazz" chords or even some weird-sounding combinations of notes.
Also, there are some very nice "spiced up" scales, arpeggios and other passages in classical music, which you can use for practice. For example, my favorite warm-up exercise is the pattern taken from the BWV 847 prelude's Presto part – it fits nicely under all 5 fingers, and then you can play it mirrored (in upward direction) for the other hand.
Another thing I like to do is to play putting accents on different notes (e.g. as if playing triples, quadruplets, quintuplets, etc.). You can also play using different rhythms, but please make sure you are actually playing the intended lengths of notes, not some mangled/lazy/broken variation of it.
For me as a guitarist they're an essential warm up exercise.
I start with the pentatonics across all shapes and positions, move on to the modes, then redo the modes using the sequence I've worked up for maximum speed. After that I'm usually warm enough not to make a fool of myself sweeping arpeggios.
Finally I play some little exercises I wrote to break out of cliches & sticking to one scale type e.g. up in a sweep, down in a pentatonic, up in a mode, back down in a sweep etc.
Scales are a fundamental part of music, and so it's impossible to avoid practicing them. It's like asking if advanced football players practice running. Well... they've been running all their lives, so I hope they know how to run. But they do various agility drills, which involve various tasks while running.
I would say that advanced musicians don't practice scales for the sake of practicing scales (recalling the notes that make them up and actuating them on the instrument). But they practice lots of things, and what are you playing if not scales?
For example, I mostly play trumpet. One of the things you need to be good at is playing with the same ease in the high ranges as you do in the mid/low ranges. One of my favorite drills for this involves playing scales that bridge the mid/low registers with the high ones. So I play lots of scales, but not for the scales' sake.
I'm also learning woodwinds. One of the first things I do when warming up on clarinet is run through all 12 major scales, full range. I'm not doing it to remember what a G major scale is, I'm doing it to build muscle memory for common musical passages. I also run arpeggios and scales in intervals for exactly the same reason. I wouldn't describe the activity as "practicing scales", but rather "practicing clarinet technique".