In reading through Alexis' archives over at Tao of D&D, I read an interesting article and ensuing discussion on training, and the difference between training and practice.
Now, I believe a starting character should be a pretty highly-trained individual: someone at the level of a journeyman, a member of a professional association, or a Bachelor of a University. In 99% of cases, the character got there through training: there was a master, and the character was a student. This could have been the Master-at-Arms on their father's estate, a scholar at a University, some weirdo who takes in orphans and teaches them to be pickpockets and sneak-thieves, or what-have-you.
Something I've never been able to reconcile with D&D, though, is stories like Charlie Parker. With a basic education in music under his belt, and some informal instruction from acquaintances and listening to records (let's say he was a 1st level jazz musician at this point), Parker set out to practice. He spent his late teens practicing 15 hours a day, and emerged as a master - one of the best in the world (by old D&D standards, probably level 9 - name level).
So, in 3 years, practicing 15 hours a day, Parker went up 8 levels. That's an average of 1755 hours of individual practice per level, or about 220 8-hour days.
How to handle a player who wants their character to "woodshed" (as in practice non-stop out behind the woodshed) for a year to gain a level or two?
I don't know, but this is something that bothers me. In an ideal world, XP should probably be awarded for practicing a great deal and revoked for failure to practice. This may be too fiddly, and would almost certainly lead to the first act of a campaign being the, "We train for two years, what now?" phase.
Don't have a solution, but it's something that bugs me.