The numbers of hours needed to achieve a certain ATAR is extremely different for everybody. Personally, I did 1-2 hours on a weekday and 3-5 hours on weekends to achieve an ATAR of 85+. In most cases, the HSC is a justice-system - those who work hard are usually rewarded.
Study consistently over the course of the year, rather than for just a few weeks prior to the trials/HSC exams. This means aiming to do a little bit of every subject every day, even if you don't have any homework/assessments assigned for that course. It could mean simply reading a few paragraphs from a textbook or doing a past paper question. But believe me, every bit counts.
Studying doesn't just mean revising; it means completing your homework, asking your teachers questions about the course's material, seeing where you went wrong in exams/assignments and redoing exam questions that you didn't achieve full marks in. Practice perfect makes perfect.
You need to be active in your learning. Don't have the attitude that everything will simply work itself out with time. If you're struggling to grasp a concept, you need to do something about it. Tackle more questions, re-read relevant textbook sections, get another teacher/fellow student to explain the concept in a different way etc. Being complacent is one of the worst things you can do in the HSC and it leads to procrastination.
Balance the workload across ALL your subjects. If you're struggling in one subject, increase the study workload for that subject (but not to the detriment of your other subjects). Balancing is a difficult thing, not just with regards to your HSC courses but also with life in general. Time management is important. Be aware of how you spend your time each day.
A lot of students fall into a trap, thinking "Why am I putting in all this hard work? I don't even know what I want to do with my life. How is the HSC going to help me in the real world anyway?" Trust me, these are very normal thoughts. This is where goal-setting comes in.
Even if you are unsure of what you want to do beyond school, setting some sort of goal can help you maintain motivation. Ask yourself these questions (I apologise in advance for the excessive cliches):
- Where would I like to be in 3-5 years?
- What career(s) do I feel passionately about?
- What do I want to do straight after high-school?
Try to think of your ATAR as simply an academic stepping stone, not something that strictly defines your performance throughout senior high-school. We are all more than just a number. So don't make getting ATAR X the be all and end all of high-school.
Hope that didn't sound too much like a motivational speaker and I also hope you can use some of the advice I gave.