What is the usage ratio of the An-225 Mriya?
As a privately-operated aircraft, this is probably not going to be a question which is completely answerable. All that is available is best guesses and incomplete information!

That said, the An-225's last tracked flight on FlightAware – as of today – was March 15, 2012 between Bangor, Maine (KBGR) and Port Harcourt, Nigeria (DNPO). That suggests that the An-225 is either being excluded from flight tracks, or simply hasn't visited the US since early 2012.

Aside from that, the only other likely place for flight information is photo sharing websites like airliners.net and flickr, or on youtube. Browsing through flickr, I tried to track down dates of recent photos in unique locations (which is a guess at best):

May 21 2013
June 24 2013
September 25 2013
September 26 2013
November 12 2013
November 17 2013
December 10 2013


I'm sure there are more, but I stopped searching. It looks like it's probably doing charter cargo trips at most a couple of times a month, which is honestly more frequently than I expected! If you're interested, I'd encourage you to track down more on the sources I mentioned.