I know, because I did that recently.
I surveyed more than a dozen friends—all of whom I know to be reasonably fast writers—and none of them could answer my question with any certainty. Their responses ranged from 200 words (Peter, I know you’re way faster than this) to 1,500.
Most of them observed that editing or revising takes much more time than writing—a sentiment I echo heartily. A few had amusing comments. One said: “Give me an hour until deadline and then the answer is ‘as many as you need.'” Said another, now retired: “I wrote the articles in my head before typing them. It saved on white-out.”
A third said: “I—or anyone—can write crap as fast as I can type…” (I don’t think she realizes how many people struggle with producing a crappy first draft, for all the reasons I outline here.)