Yacoubb,
hmm.. well ultimately everything you do throughout the year is supposedly practice for the exam. If you're asking when I started doing practice essays under time (~1 hour per essay give or take) then probably not till term two, though my school had pretty tough SAC conditions, so I had to start early to get through the year :p Middle of the year seems like a good estimate, but this will vary from student to student. If you're like me and still unsure of the structural requirements or nuances of each essay type by Term 2, you may need to spend more time doing study of a different kind. Whereas if you've had good teachers up till now, and you know exactly what you're meant to be doing, then it's just a matter of refining your ability through practice essays. Obviously this isn't all you'll be doing in any case, but you should assess your ability before jumping into a study regiment.
'Study of a different kind' was something I found much more beneficial. Reading heaps, both external stuff and past students' essays was always a big help, and I also found my classmates could offer... alternative study. A couple of us used to have long arguments about characters' motivations in out T.R. texts, or about why one approach to the context was nowhere near as good as another. Sounds weird, but for us this was a form of expressing ideas that didn't require spending an hour in exam conditions churning out an essay. Simply formulating ideas and opinions helped us a lot down the track, and this was something we could do with people of all abilities. Sometimes we might be helping someone who was struggling (best way to learn is to teach!) Other times we'd get teachers involved and force their opinions out of them
Basically, it depends what you mean by 'study.' I wasn't doing practice exams (of my own volition) till mid term three, but our SACs were almost identical to the exam conditions, so perhaps you might want to start sooner. At least look over some papers from 2009/10 to familiarise yourself with the format, then leave the others for actual content practice later.
The metaphor I always use is, like any of the maths/science subjects, you can't prepare for an exam/ do practice exams before knowing the formulae, definitions, and content. Your notes might count as preparation, and the revision you do throughout the year might be 'study' for some people, but English is one of those subjects where the way you study is entirely dependent on your skill.
tl;dr: it depends what you're ready for. Don't feel you ahve to rush it, just concentrate on learning the content and exam skills should fall into place as you go