How many years?
The HSC itself only takes one year to complete. The HSC period begins at the beginning of the fourth term of the preceding year and ends after the HSC exams in fourth term of the graduating year. During this year there are a number of internal assessments which make up 50% of your final HSC mark. The other 50% comes from your final HSC exams.
The HSC course itself begins in year 11 with preliminary HSC subjects, though these are not directly examinable.