Wrong section.
We don't know exactly yet, but we will assume Mid October as it's normally 2-3 weeks before the games release. Here's a list of previous demo releases to give you some idea when it may be.
FM2010 - Demo = 14th Oct, Game Release = 30th Oct
FM2009 - Demo = 2nd Nov, Game Release = 14th Nov
FM2008 - Demo = 30th Sept, Game Release = 18th Oct
FM2007 - Demo = 30th Sept, Game Release = 18th Oct
FM2006 - Demo = 30th Sept, Game Release = 21st Oct
From sort-it-out-si.com