for how long....well that depends on the individual.. I like to study like 4 hours a night, or on days with no school, about 7-8 hours (because it takes me that long to assure myself im learning!)
others can study for 1 or 2 hours and they're set, coz they kno everythin already
however! the amount of time studying is not necessarily proportional to how well u will do in the HSC, its how effective your study has been
being efficient by definition means learning things (actually learning, not just reading/writing subconsciously) in the least time that is possible
so to study efficiently, u need to maximise the amount of info u take in and understand where time is a premium....somehow...