According to Malcolm Gladwell's new book, "Outliers", it takes an investment of about 10,000 hours to become truly expert in a domain, like a sport or a musical instrument. Research that suggests the key to developing expertise is thousands of hours of practice in which challenges are systematically tackled (see e.g., Ross, July 24, 2006, Sci Am). I find the arguments and examples pretty compelling, and I certainly feel that there is evidence for this hypothesis in my own life. The only specific skills I have invested 10,000 hours in are LISP programming and rock guitar, and I think that I have reached expert levels in both of those pursuits. The evidence is that have made a good living as an AI professional for the last 30 years, and I almost never play in a guitar store without someone asking me if I give lessons. (I don't code any more, but I still play guitar every day I'm at home.)
On the downside, this means my piano playing has 9,000 hours to go, and my bass playing has about 7,500 hours to go. I turn 60 years old today, so I don't expect to be truly proficient on these instruments for another 10 years, although if I retire at 65 I may be able to hurry things along. Meanwhile, my jazz guitar playing is only at about 5,000 hours, and my classical guitar playing languishes at about 2,500 hours. I get some transfer among these instruments and styles, but not enough to make up for lack of practice on the specifics. Meanwhile, my hands are not what they were, so I feel like I'm in a race against time. But I find learning and playing music so rewarding that I have never given a second thought to whether or not it is worthwhile. For some reason, just listening has never been enough for me, so I hope to continue studying for a good while yet.