In forty minutes I was personally capable of writing between 800 - 1000 words in the year I did Advanced. So I would say between 200 - 250 every ten mins would be the kinda output I expected of myself back then.
Also, may I ask why you need to know how many words can be written in that timeframe? For if you are planning your essays by this, then its a very rigid perspective. It would in truth be a better method of preparing your essays by looking at whether you have fully explored your concepts or not, as quality is more important than quantity, for an 800 word essay that is highly relevant and insightful will score better than a 1500 word essay full of garbage.