180 days from check in date. They go by calendar days so if you are checking in April 30, 2009 the 180 mark may not be on October 30.
Use the ADR calculator here on Intercot to find your exact 180 mark. If by chance you call Disney Dining and it is before the "technical" 180 day mark they will let you know the correct date to call back.
[COLOR="Red"]3 trips prior to 2000
2006-BCV & WLV
2007-WLV & BWV
2008-CBR & SSR
2009-WLV, YC & Disney Wonder
2010-CBR, POLY, SSR & Disney Magic
2011-ASMu, Disney Dream,SSR, CONTEMP,POFQ