I havent been through the process, but I am of the understanding that to convert an ATPL you would still have to do an ATPL flight test in australia, and have completed an MCC course and the conversion exams.
I think it comes down to CASA's assessment of your experience as to how many hoops you have to jump through.
And with regards to how long - CASA takes 6-8 Weeks to process a licence application at the moment... so you're probably looking at that window just to hear back from them once you have all your paperwork in order.