I have to agree with the 5 year estimate, there are a few factors in play to support this but the biggest is the gaming industry itself. There is a huge movement to creating games that are cross platform, the days of PC exclusive are rare. To create a game cross platform the game makers built toward the lowest common denominator, the consoles. Now even if the next generation consoles hit they will still, based on the info out so far, be a generation of so behind the current PC and then not even close to a high end PC setup.
This can be seen already today with single card setups where people are using video cards that are close to 4 years old still and gaming with them just fine. They may not be running top of the image quality graph but they are still using very respectable settings and getting great game results. Heck I play Skyrim at my in-laws on a GTS 450 at 1080 and that is a low end gaming card from almost 3 years ago. Your talking about dual top of the line cards, 5 years should be easy.
However as always take everyone's opinions on this with a grain of salt. This industry can suddenly surprise you and we could see something amazing in the next 6 months that changes everything.