If you practice the trombone for 2-3 hours weekly (six half-hours, whatever), you will slowly learn the notes and some rhythms. You can develop a fairly nice midrange sound if you simulate a good example, like a teacher. You can have fun. Many beginners, junior high trombonists, and some high school players practice this way.
(And I’m not counting ensemble rehearsal time in this. It does not really count. Well, yes it might build your endurance, you can memorize the field show, and you learn a lot about playing with other musicians, how to act, how to follow a conductor, and how to take directions. But this is not the same as the skills gained in the practice room.)
If you can get the practice hours up to ten, week after week (40 a month), you will notice some important and valuable developments in your playing. You will become more “fit.” You can handle 5 or 6 books at a time, or more. There will be more time to regularly address things the Little People often neglect: air exercises, tunes by ear, high and low range, some jazz, recording yourself, clefs, the outside keys, real sight-reading, duets with peers, tough etudes, audition materials, orchestral excerpts, jury solos, vibrato, quality time with pianists. Your reading will really improve! You won’t be sore the day after a big blow. You can use the metronome, mirror, and tuner properly. Do dozens of routines of flexibilities, scales, arpeggios. If you find something really hard, there is time to work it out and work it up. There will be time to solve any bad playing habits. You will be thinking about trombone while you sleep! You’ll be quite proud of your playing and your progress. You will deserve to Get Some Work.
If you will develop the stamina to really practice 15-20 hours a week, then you get All Of The Above PLUS you’ll tear through the literature much quicker, build a repertoire after a while, learn tunes and the changes, progress quickly with unusual techniques, review old material, be a serious competitor at professional auditions, and much more.
If you cannot do this . . . well, the benefits will be elusive. Know that there are students all around the country practicing 20+ hours a week. You will meet them, at the audition. There will be one winner.
(Some other instrumentalists will find these numbers a bit low, and maybe they are low. Ambitious pianists and string players devote much more time to practice, because they can.)