The best way to ensure you're staying on top of things is by spending a couple hours on yr 13 work and an hour or two going over yr 12 work and doing questions. Depending on how much time you spend studying each day you could do that all in one day or split it over two or three if you're overwhelmed and may have exams and homework. I'm currently going into my first yr at uni in a couple weeks and i can tell you surely that september is going be hella stressful with getting back on track and adapting to yr 13. Its a major step up from year 12 so the first month you should focus on doing homework, coursework, start thinking about your personal statement and just settling back into school life really. I would strongly encourage you to realistically start working on yr 12 stuff on top of your yr 13 work by october or latest november. More importantly don't stress too much, try and manage your time to the best of your ability.