I entered Germany in June, it was a visa-free entry (because I have an Australian passport) and I stayed 66 days before exiting to UK. Can I now return to France and stay 24 more days, or do I need to wait until the 6 month period is finished and 90 days resets? I'm not sure whether I am entitled to multiple entry or single entry, because of the Visa waiver for Australians all I have is the date and stamp on my passport from Frankfurt.
If you have a visa, it's either single-entry or multiple-entry. If you have visa-free entries, it works as if you were getting a new visa each time you cross the border. You can visit multiple times, as long as your total number of days of presence never goes beyond more than three months in a half-year.
This site answer your question: http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=1841750
The official rule for the Schengen visa is that, on the day you first enter any country within the Schengen zone, you start a 180 day "period", within which you can stay a maximum of 90 days in all Schengen countries. After you have racked up your 90 days (in one go, or in several chunks if you leave to non-Schengen countries & come back), you can't enter any Schengen country again until your 180 period ends. And on day 180, no matter what, you have to leave Schengen territory (even if you haven't used all your 90 days), but you can then turn around & come back in again the next day & get a new 90/180 day period. So: if you enter on day 1 & stay for 90 days immediately, you have to leave on day 90 & cannot re-enter any Schengen country until your 180 day "period" is up.
I hope this will be useful to you.