It varies so much that there's no way to really tell. The same country can have an applicant in the waiting game for a year and a half and also have someone apply after that person and get the visa within a few months. It really feels like luck of the draw. A US applicant applied for the 300 visa two days before I did and we both got the grant around the same time. But when we both applied for the 820 around the same time, mine was granted within a month and last I heard from her she had been waiting 4 months and still no 820 grant. Didn't speak to her again after that so no idea how long she ended up waiting but it's a perfect example of two people from the same country applying for the same visas at the same time and having two completely different wait times.
It sounds like the recent changes to the process are intended to speed things up, so hopefully that's the case.
