I would say around 900-1200 or thereabouts. But the main thing our teacher keeps stressing is that even if your story is short and "punchy", then you can score really well. Obviously you still need a certain amount of length to develop a degree of plot / show understanding of conventions / etc but you don't necessarily need to go in there and write 15 pages worth.
That said, I am really worried about the creative writing part of the exam. I really hate the idea of being assessed on stories that have to adhere to a set genre plus whatever stimulus they give you.
In the context of the exams I see why they do it but for me it defeats the purpose of "telling a story" if the writer is forced to try and commit to a certain fashion of writing.
You can go in with prepared stories but often you'll have to just wing it for huge sections in order to make it relate to what they give you. i.e. they may restrict you to using a "significant setting" they give you or something, as per previous exams. Very scary to think that as much as I've prepared, I might just suffer a creativity blow when I get in there and not be able to be "creative" enough to make a story fit with the stimulus under pressure.