Hey apologies for the late response (i may have been on holiday 😃)
In terms of the written part of the exam, make sure you are confident with your prac report skills. i remember for one of my sections, they gave us an introduction (with background info) and a methodology of a prac, and they asked us to write an aim, hypothesis, discussion and conclusion which suited that information.
Make sure you know:
Independent and dependent variables
Precision and accuracy
Validity, reliability and repeatability of an experiment
This content can be found in any VCE science textbook (generally chapter one).
In terms of time management, I found the test to be fairly manageable (no where near as hard as the melbourne high/macrob test) because the philosophy of the principals is to find students with a genuine passion for science and learning, so they make the interview more difficult instead to help find individuals with these traits.
As for the confidence in your ability to do well enough in the test, I remember going through ATAR notes forums when I was in your situation a few years ago. I read something that kind of stayed with me which was "if you have gone to the effort of finding this discussion page, you are already showing more determination and enthusiasm than most of the other applicants. This part of your character will shine through in your application, so while this process is stressful and difficult, you'll get in if you have what they are looking for."
When I was preparing for the test, I used Strive for Excellence. Looking back, they weren't great because their questions didn't simulate the questions on my test very well. However, doing mock exams under timed conditions did give me experience and confidence going into the real thing, so I would recommend doing some prep that aims to simulate the real experience. As long as you have a good background knowledge in year 8-10 maths and science, you will be more than ok 🙂
Oh and in terms of when you should start your preparation, I guess any time from now really. If I were to study for it again, I would ensure my fundamental maths and science skills were good, and then put a lot of emphasis on my prac writing skills right up until the actual test.
Let me know if you have any more questions, and good luck for your application. My email is firstname.lastname@example.org if you would like to ask anything more personal 🙂