and it's from your rating date.
So for example if you passed on 1st June 2008 you would have to do 12 hours (one of which is with an instructor) plus 12 take off and landings between 1st June 2009 and 1st June 2010 to revalidate (?) due to experiance. If you don't it's a retest to get your rating back, but I don't believe it is a full flight test.