There isn't any, you can drop whenever, whatever you like till you have 10units with 2u of english remaining, but you'll first have to get your head teacher and parents to agree by signing a subject drop-out paper.
However you can't pick up a new prelim course after your subject selection, so be sure you are certain which subjects you want to do since these will be the courses you'll have to pick from for yr12.
Deadlines are done by the school so they can arrange the timetables and to pick whose doing what subject. There isn't really any rules to it.