Are you in year 12?
If so, START NOW. NOW. NOT LATER. NOT TOMORROW. TODAY!
It's better to do a little bit everyday, for a long period of time. Than A LOT everyday for a short period of time.
You are going to be exhausted, not have enough sleep and not learn effectively. (And It will be really stressful)
Also, don't count study time in hours. Cause (well for me), it never goes to plan. Even for practise exams. Even though there's a set time complete it, sometimes I finish really early, or sometimes I am stuck on one particular question that takes ages for me to wrap my head around and solve (even after the times done)
Just set yourself realistic goals for you, and try ensure you complete them. Don't make timetables too rigid, (everyday from 3-5pm etc... ) cause plans don't always go accordingly. And the trick is being able to be flexible, compromise.
However that now exams are just around the corner, you definitely need to get into revision mode ASAP. It's not too late to get the scores you want. There's still time. But depending on what you do between now and the exams, your scores and ultimately ATAR could be very very different.