you wait until you have 2 units that are clearly the worst for you. then you make your decision (personally, i didn't drop to 10 until a week before trials (i dropped legal)).
Re: Hsc in one week
i struggled a bit with 2012 (could only get the first 4 marks of 16.b) but other than that i usually drop 3 at the most! except for 1 year i can't remember which one that had a complex polynomials q16 where i couldn't do the last 5 marks (it was so hard i can't even ffs)