... A meaningful standard of fluency for me is the ability to read an artistic work of literature such as a novel by a difficult author like William Faulkner or James Joyce. How many native English speakers can do this? How many do do this?
I call it "the airplane test": take a 400-page novel of this type on an intercontinental flight and read it cover to cover by the time you land. If you remained engrossed and enthralled the entire time, you really know the language. You need to know about 20,000 words in order to do this, while you only need 10,000 for sophisticated conversation, and far fewer for daily needs such as business or study in a specialized field. In order to read a novel "fluently," you should not need to have recourse to a dictionary, but you should profit greatly from every single word you do look up if you choose to use one.