Mine was only a few weeks, 4 or 6 or something. The police check in the local auth I needed to do it was the thing that dictated the timeframe. I submitted the application with medicals and police check to follow and basically as soon as they got those I was sent an approval email. I didn't use an agent, just made sure the papers were all in order, scheduled nicely, and a good lot of evidence. We send family Christmas newsletters,so I included one of those for each of the years we've written them, and that doubled as our own evidence as well as photos, and of course copied a few anniversary cards to each other and from parents etc to us.
I worried a lot about it but when I did it, it wasn't half as bad. Oh, I also got the childrens' Aussie passports for the 2 that didn't have them, before I submitted my application. Being married over 5 years,and having children as well, really made the application a lot easier I think.
Can dig out the stuff if you have any more questions. HTH