Originally Posted by Benetanegia
GeForce 7950 GX2 - June 5, 2006
GeForce 8800 GTX - November 8, 2006
5 months difference
GeForce 9800 GX2 - March 18, 2008
GeForce GTX 280 - June 17, 2008
GeForce GTX 295 - January 8, 2009
GeForce GTX 480 - March 26, 2010
14 months -7 months of GF100 delay == 7 months
GeForce GTX 590 - March 24, 2011
GeForce GTX 680 - March 22, 2012
Average 6.75 months, ~7
GeForce GTX 690 - April 29, 2012
And since talking about intentions, what could or what a company is willing to do, rather than actual releases, we might as well go with the 3 month or 5 month figures. Certainly Nvidia was willing to go with a dual GPU back then despite the short time until the next gen.
But we're talking about the 690 in this case supposedly being replaced by another kepler card with the rumored GK110 780. In all of the examples you listed you have dual-GPU cards being replaced by single-GPU cards from an entirely new generation, in most cases bringing new feature sets also.
Nvidia releasing the 690 before the supposed 780 would be like if they officially released something like this
right before they released the 480. There is a reason you only ever saw cards like those from 3rd party board partners.