Some sources suggest that if you're aiming high, you need a minimum of 45 hours per subject revision time as well as spending time working through past exam papers. This will help keep your stress at a healthy level (yes, a certain amount of stress keeps you motivated).
So its a good time to start to
Elevate Education's website provides exam prep, sample questions and revision hints, subject based revision. Click here for the student portal
TSFX also have a range of free resources through their "Edge Online"
Please note: websites and resources are provided for your information only. This is not a comprehensive list and the school does not endorse any particular organisation.