I've heard by some that the deadline is before the half-yearlies, and others say you can drop before the first day of the HSC exams, when am I able to drop?
You can drop a subject whenever really, most people say to drop before half-yearlies because studying a subject for 2 terms only to drop it is really inefficient
You can drop most subjects any time before your HSC exam.
The exception is Ext 2 Maths, which can't be dropped after your school has submitted your assessment marks.
Even then, you can't drop if your school has no way of assessing you for 2 unit (ie. you haven't done 2U tasks).