Probably when you've found that you don't have to read the documentation for the base language in a while. Everyone constantly reads docs on infrequently used

Probably when you've found that you don't have to read the documentation for the base language in a while. Everyone constantly reads docs on infrequently used libraries and for gotcha parts of a language, but that's far different from having to look up how to create/destroy objects, perform all conditionals, use the data types, etc. Equivalently: when you wouldn't be embarrassed in an interview to write something on the board in that language.