It depends a lot on your measure of what constitutes clear skies.
North of the divide, success rate for good clear nights (less than 1/8th cloud cover for >3/4 of the night) is around 30-35%. In Melbourne that would be around 20-25%.
However, if you are after just two hours of mainly clear skies for some casual observing then those figures are substantially higher.
This sunshine chart gives a good indication of how it varies geographically:
http://www.bom.gov.au/watl/sunshine/
Phil