(When I was about 19) It took me about 4 months of about 1 hour practice a day to learn Paul Gilbert's famous picking lick to play it totally effortlessly at high speed. Took me about a month to have very good control of dynamics of it. I had very good left hand technique and I could tremolo pick forever with zero fatigue and minimum movements (that took me about 4 years or so, I have started "late"), 4 months were spent on string crossing and dynamics. Now, I didn't do any of this metronome 5 bmp per day nonsense, or I'd be still playing this lick at 90bpm. I basically figured it out first how to play it fast (mostly by short high speed bursts and going by the feeling of tension), then slowed down to get it in control, then did some metronome practice. The technique is different when playing at mid/slow speeds. The major thing was playing what felt (no tension) and sounded right. Playing funk rhythms at moderat/high speed with a drummer or metronome helped a ton to loosen up/get good sense of accents. From real live observation, a person with good, minimum tension medium speed basic technique, good ears and rhythm sense, will probably learn high speed playing in about 3-4 years of 1-2 hours a day dedicated, focused practice. The trick is that many, many people never figure out what is the right fast playing technique for them and they get stuck (you can't do it with metronome 5 bpm per day thing - try sprinting very slowly, see what happens ). it's extremely hard to teach and explain, but some people get it right away, usually by accident. Maintenance is another story. Paul Gilbert once said that fast picking is like tropical fishes. They look pretty but require a lot of constant effort or they die.