Usually it's from mid to late August to mid to late September.
I don't want to give exact dates because it's been shorter and shorter each year
2006
Free Dining - August 13 - September 30
2007 Free Dining - August 26 - September 22
Some experienced a slightly longer window as they are from the UK or participated in bounce back dining, which has a longer window.
Look for general free dining to be announced early April, with booking beginning around April 11th