It depends on your school for Trials.
We had ours starting in week 5 of term 3, and going until week 7, I think. That was with some assessment tasks the week before as well, plus compulsory attendance. No stuvac, excepting the days during the exam period when you didn't have an exam.
As for HSC, we had the two weeks of school holidays between term 3 and 4, plus the first day of term 4, which was technically the only stuvac we got. So effectively 15 days - not counting days when you don't have exams.