Ok so i've only done 3/4 sciences so far, so this may not be entirely relevant to other subjects.
The post above me explains when you should study - and the checklist idea is very good

however to use that you need to know what to do as study. Simply rereading textbooks and writing notes is not effective study.
The first point I wanted to make is that you should never not be studying - I'm not saying you should be studying every night, but see all of your classes and homework as preparation for the SAC - because it is. Don't tell yourself that its okay that you don't understand it, because you can study it later. Find out what it is now, you should not be learning anything new when studying. You shouldn't have to try and remember what you studied first, because you should have learnt it in class. During class/after school is when you should make all of your notes, when you first learn the content,
rewriting your notes is a very ineffective way to study.Timing of when you start studying is up to you, it depends how well you already know the content that is going to be on your SAC. If you don't know it very well start earlier. Adjust this as you go through the year, if you do badly on a SAC, start studying earlier for the next one.
What I did was go through the content that would be assessed on that SAC and create posters (obviously this won't work for all subjects). My posters did not have much writing, they were just very quick visual representations to sum up the content (I've attached photos of some of my biology posters to the bottom of this post so you can get an idea). As soon as I had made these, i stuck them on my bedroom walls and they did not come down until after the exam (in fact, some of them are still up - I really should take them down now lol). This meant that I was regularly exposed to the content. Waiting for a computer game to load? Look away and glance at the poster. Every time you do that it jogs your memory, and will help you remember it faster during your tests.
This was the only form of 'writing notes' I did as study.Other than this all I did was practice questions, for biology I had checkpoints and the ATAR Notes topic tests - Both of these separate questions by topic, so it is easy to find questions relevant to the content you are doing. If you want more questions you can try asking your teachers for previous years SACs, some won't give them to you but some will.
One other point - One of the best ways to learn is explain things to other, jump on the threads for your subjects and try and answer questions - If you can't answer someones question then it is something you need to study.