Hmm, well it does kind of depend on what subject you are preparing for, and whether you are in year 11 or 12 (@fridge).
Personally, last year I did 25-35 practise exams each for methods and physics, beginning ~2 months before each exam (remember I had a mid year for physics) but that's just me.
It also does nicely tie in with my next point, if you are studying for a subject that used to have a mid-year (the sciences + accounting) then I would do unit 3 practise exams as soon as I finished unit 3. For me, this year I have done some 5-6 unit 3 chem exams. This is especially important because some unit 3 subjects are completely different from the unit 4 (eg physics), and you need to do the practise exams in order to effectively remember the unit 3 content.
My next point is checkpoints. Again, with my anecdotes: I actually have 3 checkpoints for spesh haha, 2013, 2009 (finished already) and 2004. A bit excessive I know, but the overlap of the three books contains pretty much all the past vcaa qs from 1994 - 2012. I'm completely each section of the checkpoints book (in order of topics we're learning). This gives me a good idea of the exam-style questions.
English is a bit different, I personally would NOT do endless practise exams. Instead, I would practise planning responses, 5min per prompt since I find this more productive than mechanically writing essays. But that's just my personal view!
Finally, just two last point on practise exams:
1. To them under time conditions

2. KEEP A RECORD OF YOUR MISTAKES!! Right it down in a book, or on a specific sheet of paper!! This makes sure I don't keep on making the same mistakes and learn from them!
And good luck!
