For me, I did 2 hours per night on average.
I did also spend 1 hour a day exercising (Weight training or mountain biking) and a few hours of music (School bands and solo work) a week. This was, i think, essential to my success (I scored well above my goal) as it kept me fit, relaxed and gave me a break from bookwork whilst still engaging my brain.
Having said that, I spent ever spare period and half an hour of lunch in the library working!
...I also didn't start studying properly (ie not cramming!) until year 12.
If i can offer more advice, It is to keep well ahead with your studies. If you start preparation for a sac a few weeks out, making sure you understand all the principles and ideas that will be examined, you will relieve yourself of a lot of unnecessary stress and will reduce your workload!
Remember guys, Stay human! See your friends, do things you enjoy and stay organized.
Good luck!