About as long as it takes for a weight lifter to notice that he's building muscle. Progress doesn't take much to show. I noticed reasonably fast that my fretting was getting better and I wasn't dropping the pick nearly as much pretty early on. As far as anymore, well, when I began sweeping it certainly wasn't what it is now, so the progress was quite blatant. In basic, if you apply yourself and stick with it, the progress won't take long at all to show. And the 4 years to hit 220 bpm? Bull****. If you sit and practice and dedicate time to it, it takes hardly any time. After a shade over 2 years of literally never practicing with a metronome I found I was having a hard time at 100 bpm. So I sat down and dedicated a few hours solely to practicing with a metronome and within 2 days I was crankin 16th notes at 200 bpm. How was this possible? Well, when I found myself having a hard time keeping up, I just slowed the tempo back down to something I was more comfortable with then tried again. When practicing new chords and chord changes, don't just look at the fingering and try to nail it, watch your fingers go where they go vs where you want them to go. The primary thing that takes time is how you do it. If you do it "wrong" then it's going to take more time and probably cause a bad habit or 2. if you do it "right" then it'll come easier and more comfortably as well as permitting other things to come just as easily. How to prevent doing things wrong is as simple as asking a question to an educated person, reading a book or even playing guitar with someone better. Observance would probably be one of the greatest keys to learning. At least I'd think so. Helped me a bundle.