good luck michael,
open days can be a pain.
no one else is talking so i'll have a stab at it.
best case scenario would take you 3 days, providing they only pick enough people for 1 group day as sometimes they have 2
then in the usual 3 days of interviews, you can try and pick your spot, providing someone doesnt grab it first. just tell them you came from melbourne, or possibly, threaten them with a knife or some sort of armlock
worst case scenario, well, 2nd worst one, due to the fact the worst one would be not making it past the open day. would be 7 days? if you include open day, 2 day group, and 4 days of interviews and you get stuck on the last day. though the chances of this happening are slim to none.
anyway, back to my point, 3-5 days would be fine. but most likely just 3-4 maybe good to have an extra day handy? go site seeing? the beach? look for romance? something like that.
hope it helps.