The date comparisons don't mean much tbh.
You could also look at how quickly the dual cards followed the single cards.
GTX 280 = June 2008, GTX 295 = Jan 2009 (7 months)
GTX 480 = Mar 2010, Dual GF100 not feasible...
GTX 580 = Nov 2010, GTX 590 = Mar 2011 (4 months)
GTX 680 = Mar 2012, GTX 690 = Apr 2012 (1 month)
Using date comparisons you'd expect the 690 to have appeared a lot later in the year. I think we should relate to W1zzards post about CeBit 2013. He's one of the few people here who doesn't speculate.