Also, if you know your essay well enough and have refined it throughout the term/week, you already "know" it so it really only takes 2-3 hours max. to get little details right and memorise it down to the word.
Agreed - a lot of it depends on how familiar you are with the response. Generally speaking, if I have recently written an essay, it takes me about an hour to memorise it sentence by sentence and about two to have it down really tight (this is for a 1,000 word response). For the few I have done in uni, a sentence for sentence memorisation was sufficient.
Also, the more experience you have with memorising the better you are at it - in Year 10, it took me around 3 or 4 hours just to learn 400 words worth. By the end of year 12 I could do 1,000 words in about an hour (which has stayed the same since, because I dont really do memorised response nearly as much anymore).