4870 - June 2008
5870 - September 2009
6970 - December 2010
7970 - December 2011/January 2012
R9 290X - October 2013
So the R9 290X is the only real anomoly in that, with the other cards all being about 15 months apart (note the 7970 paper launched in December, retail parts weren't available until January). Then again, when you look at nvidia...:
GTX280 - June 2008
GTX480 - March 2010
GTX580 - November 2010*
GTX680 - March 2012
GTX780 - May 2013*
GTX 980 - September 2014
They've been on a > 18 month cycle largely - remembering that the GTX 580 was basically a respin of the GTX 480 - the design was almost identical, they just tweaked it to get better yields - and the GTX780 was basically a rebranded Titan, so they messed around a bit (a lot?) with the cards and chips throughout the 600/700 series. The difference could be explained by AMD largely being behind in the single GPU stakes for much of that time so accelerating their development cycle?