For Algol-like languages it takes me about 5,000 lines written and 20,000 lines read before I feel comfortable with it. A good way to do this is to re-implement some pet program in the new language. That way you don't confuse the work of a new problem with the work of the new language.

[edit] I second far33d. You can write FORTRAN in any language. :) Each language has its own conceptual style, idioms, and so on. Learning how to think in a language is real fluency.