Tamee
I believe the time starts from the date on the results of the police check itself.
But don't panic, I understand that immigration accepts most of them as being valid for 12 months - otherwise everyone going through this looonnnngggg process would be doing nothing but getting new police checks, right?
In all my readings of this forum and many others, the only time I've seen people being requested for new medicals and/or police checks is when the process drags on past the 12 month mark.
Hope that helps.