He's just saying that the actual essay writing component of this shouldn't be a long term process. Most of the regular assignments we receive now require 1,000 to 1,500 (well most, I know there are assignments where I've written essays of around 3,000 words), in which most are composed in just a few sittings. The bulk of this major work will involve researching the background and historiography into your thesis, and then another long process will be the annotating at the end and the alterations that occur after the essay is written.
I'm not too sure if 'everyone' can write a decent History Extension Major Work in just three days however. So how 'bout we just not leave it to that stage, and all start early. n_n Then, hopefully we shall all be calm come Term 3.
Just try and spend as much time you can on your Major. The more research and effort you put into it, the better it will become. Also, remember to listen to the advice given by your teacher and follow their directions when it comes to the drafting of your essay.
And most importantly, Good Luck all. n_n