A long time. Much longer if you go the long way or if the wind doesn't blow. Seriously, this isn't a question that can be answered with any accuracy. Too many variables.
How long have you got? Are you looking for a quick delivery ie you're Brisbane based and you've found a yacht in Perth. I went the other way (Brisbane to Perth) taking 6 months. Another forum member did the same trip in 3 months. It can be done in less time, or it could take longer.
