G'day
My tip is in reference to Yr 12 actually (i thought i'd put it in here, as u guys will b there soon, and prolly would wanna know this in advance!)
If you're looking for good good marks in HSC, & u're willing to put the effort in to achieve it, a good guideline would be 15 min / unit / day.
ie. if u're doing 2U english, allocate at least 1/2 hr worth per day.
This is ofcourse, in reference to studying (ie. revision / reading ahead), NOT homework.
If u're doing 12 units altogether, it means 3 hrs of solid study each day.
I know this seems like a lot, but I can assure u that if u're willing to put the effort into doing this, u would not be able to regret ur effort in yr 12!
Also, CONSISTENCE is the key! i know it's hard, but try keep it up with the study - a little bit each and every day! If u can get it into a habit, then u're most of the way there! =D
Good luck! =)