One of the things I hear most often from students is "How long will it take me to get good?" Unfortunately, the answer to that question depends upon a number of variables, including 1)what type of music you are learning to play, 2)how long you practice for each hour/day/week/month/decade, and 3)what your definition of "good" is. There should be some joy on the musical road, as you hear yourself improving each time you approach the instrument. If you're just bored or annoyed by that answer, however, here are some tips that I have found helpful over the years:
1. Practice to your weaknesses. It is a lot of fun to play through the part of a tune that you know well, your fingers dancing over the keys or strings in a mesmerizing blur- up to the point where that really hard chord comes up and you have a musical train wreck every time you run into it. That part is a bummer. So, you have 2 choices. The first choice is to play the fun part, mangle the difficult chord, play the fun part, mangle the difficult chord, ad nauseum, which will be an enjoyable (mostly) way to spend time with your instrument. You will only improve the difficult section very, very slowly, however. The second choice is to ONLY play the difficult section over and over and over and over until you are able to play that part as quickly as the easier section. This may require a nose-to-the grindstone mentality for a short while, and you may dishearteningly find that the end result is that you have to slow the easier section down to get the whole tune the same speed. It is a great freedom, however, to not have that difficult part hanging over your head any longer.
2. Practice SLOWLY. After you've been playing a certain piece or scale for a while, it moves from a cognitive exercise to one of muscle memory. The better trained your muscles are to move a certain way, the more precisely they will recreate the exact same movement when called upon to do so. If there's a certain place in a piece you're working on where you sometimes nail it and sometimes flub it, this is just the kind of thing that will help you nail it every time. Try it sometime. Play a piece of music you have memorized EXTREMELY SLOWLY a few times each day for an entire week. No cheating! Then, after your weeklong toil, Play it as fast as you can. If you've been dilligently practicing, your speed will have improved dramatically over just one week. Try it, it works!
3. Practice your least favorite thing first. As you sit and concentrate on your music, over time your brainpower will wane. If you wait until you're sort of burned out from practicing to play the exercises or piece you've been dreading during your whole practice session, you aren't going to get very far with it. By saving your favorite piece until last, you'll get a little boost when you begin working on it, which will help you to work at it a little longer.