Personally I don't think it's the quantity of study ur looking for but the quality-if u do subjects u enjoy then u can learn them more effectively(usually) without toommuch time or effort e.g for phys,chem or modern i didn't make any study notes or take notes in class-reading the sheets and the textbook was sufficient(we gotm lots of sheets tho). Maths u should do at least for half the week cos that requires the most practice. i studied(this isn't a comparison here, just what worked for me) about 1 hour each weeknight, however on the weekends, i would spend about 8 hours in total-however i did not study with an ipod or any other distraction. and when u study make sure u understand it-don't just try and memorise dot points-i didn't know what was in the syllabus, i just read the textbook which i knew would deal with the syllabus and read through it cos it made sense. So study effectively. If i had the choice i would have studied more, but i lived far from my school and would reach home around 5pm-plus with working out and other activities my time would often run out.
Mind u, in my olympiad preparations and extension work in yr 10, i had covered a lot of topics in the hsc(mainly for maths and chem)