When you stop trying to master it?

That has a nice ring to it, but I'm at least mildly serious. A novice tries to write Perl code that looks like other languages he knows. An apprentice works Perl idioms into his code, and a journeyman uses the idioms correctly. A master stops noticing any of that and just writes code.

See also The Tao of Programming for a humorous take on mastery, which isn't quite as humorous if you know what you're doing.