A couple of axioms here. Firstly, most people agree that you should write your first draft quickly, without too much care on quality, and get something down on paper first. The real writing begins with the second draft. This is the approach that most writing books try to teach, and I'm taking this as a given for this question.
Secondly writing is more than just typing (no matter what Truman Capote says), so there has to be some level of thought into the first draft. I'm no professional typist, but I can at least manage 40wpm when copy-typing. Clearly though I'm not going to reach 2400 words per hour on the first draft.
So what sort of figure should I be aiming for? At the minute I'm around 250 words/hour which I'm worried is veering into 2nd draft territories for slowness. Are there any de facto guidelines here from the professionals?