it's going to take approx 35 minutes right? so I guess it depends on how complex your argument is, how fast a writer you are, and if you come in prepared and knowing what you're going to say. since I honestly don't know what to do for the essay we bring the question in ourselves, that last bit is what i'm probably most afraid of lol
i'd say 500-600 words is ideal, but maybe 400 would suffice if pressed for time. I think the key is to stick to a few key points, be concise, and state everything that you think is relevant in an organized fashion. If that's done, then it really doesn't matter how long it is; quality, not quantity =]