Oftentimes it depends on execution. For a 50 mark questions, the minimum would be a page and a half. Yet, for a three hour exam, if you're solely doing six questions, I'd ensure you breach the two page mark. If it's a siege or battle, there's going to be a section that wants you to expand on his character -- that can easily, and quite successfully be padded.
But I'd worry about specificity, not length.