R420 - June, 2004
(16 month gap)
R520 - October 2005
(20 month gap)
R600 - June 28, 2007
(12 month gap)
R700 - June 2008
(16 month gap)
Evergreen - September 23, 2009
(13 month gap)
Northern Islands - October 22, 2010
(15 month gap)
Southern Islands - January 9, 2012

Now taking into account the fact that ATi/AMD has NEVER released two product generations less than 12 months apart, with most releases averaging a 14 month product cycle.... I highly doubt we'll see anything different this time.