It is up to the individual to prepare as much as they want during summer. Everyone is different, so you have to find what works for you. Also note that at the higher end of scores, hard work does not necessarily correlate to better marks. I spent most of my summer before VCE doing Methods. I completed over half of the textbook, and yet it was my worst score for the year. I think I read somewhere in the thread that if you've worked during the summer, then you may slack off a little during the year. There may be some amount of truth to this. Although then again, I hardly ever remember doing any work in class for all of my subjects...
To contrast studying over summer, for Biology and EngLang I did absolutely nothing before starting, and they worked out quite well. At the end of the day, you can know the course inside out, but come exam day it is not going to help you if your exam technique is not well planned and efficient.
With regard to burning out, I think it can and does happen to all of us to an extent...but, I don't really think it should affect anyone too much. Even if it does, I think the importance and high stakes of it all should be enough to get you through. Sorry for the boring old cliche, but I guess to an extent when the going gets tough, the tough get going. :p
All you have to do is keep thinking of that final date when freedom will be yours, and go like hell until then. After that, you can rest all you want. To ease off beforehand would just be a waste.
That having been said, I think you really need to be fresh for exams, so definitely no intense study or late nights during an exam period. Try and knock off all of the hard stuff at least a week before your exam.
