One of the things I hear most often from students is "How long will it take me to get good?" I answer that it will take them exactly as long as it will take them to learn Latin.
Yeah, that is a complete nonanswer, but in fairness, it's a nonquestion as well. The superexact failsafe directions to musical goodness are contingent 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. The goal is to enjoy the process, to enjoy hearing music come out of your fingers that used to just come out of your stereo, to feel joy in wringing every last bit of bittersweet reverie from a simple Mozart minuet or every bit of angst from a Fall Out Boy tune. If you're just bored or annoyed by that answer, however, here are some guidelines 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 two 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 a mostly enjoyable way to spend time with your instrument. You will continue to have musical train wrecks every time you play that piece, 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. I usually start a section like this at half speed and slowly creep it up until it's moving along at the same tempo as the rest of the piece. This will require a nose to the grindstone mentality for a short while, but the benefit of being able to play the whole piece trainwreck-free is well worth the effort.
2. Practice SLOWLY. After you've been playing a certain piece or technical exercise 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 piece that you enjoy playing but are sloppy with, 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 three times each day for an entire week. No cheating! Then, after your week is up, 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 getting burned out from practicing to play the thing 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 an emotional boost when you begin working on it, which will help you to work at it a little longer.
4. Just do it. The hard part about practicing is not the act of practicing. The hard part about practicing is sitting down at the piano (or guitar, or computer, or easel). You may have to muster up some willpower to get yourself to the piano, but if you can do that for a few days in a row, the principle of inertia will begin to take over and it will get easier and easier as the days go by. If you are mindful when you practice (that is, practicing while you practice as opposed to mentally constructing the shopping list while you practice), you will be amazed at your progress. A dilligent twenty minute practice session every day is much more beneficial to your long-term musical development than ignoring the piano for a week and spending nine hours banging on it on Saturday.