Wow, good on ya, you're starting early

I got a 50 last year and I got through about 30 past exams for each unit, but I only started doing prac exams for unit 3 about 3 weeks before the exam, so end of May. Having said that, I would do exam style questions throughout the year (checkpoints, studyon etc) so when I did start doing exams it wasn't completely alien. Also, I wouldn't say that any number of prac exams will guarantee you a good score- it's how you use them. Make sure you learn from your mistakes, maybe write down questions you find hard or got wrong and see if you can still tackle them closer to the exam date.
And of course have a thorough understanding of all the concepts- know them well enough that you can explain them to someone else in your own words.
If you do all that you should be set, good luck
