I think this is related closely to another popular question: how do you determine that someone else has command of a particular language?

I think the point that I would pick is when you start feeling constrained by it. If you start wanting to do things the language can't do, or can't do well, and you can explain why it can't do it well, then you have explored pretty much all of it.

That's why my favorite interview question is "So you're applying to program in FOO. What's something you hate about FOO?"