this topic brings up two thoughts for me:
one based on the "how much practice time" thread is:
you are a musician when your performance takes up
more time than your practice.

the other is a thing i remember hearing recently about the diff. between
amateur and professional:
an amature practices until they can play it without making any mistakes.
A professional practices so much that they will not ever make any mistakes.
(something like that...not sure i got the wording right..did I hear that from mudcat?)