Currently sitting on a GTX 670 2GB, which I picked up in March as an upgrade over my GTX 570. Stuck it in my system with an i5 2500K @ 4.3Ghz, built in March 2011.
Ideally I'd be interested in the 800 series next year, but price and performance are a big factor for such an early upgrade. I waited two years go to from an GTX 570 to a GTX 670, and if I'm not in a rush I could end up waiting for the 900 series, repeating the current two year gap.
This all assumes I stick with team green though. If AMD put up a good fight I could make the jump to them. I'm also pretty damn concerned how long my CPU is going to hold out before it's severely bottlenecking my GPU. It would be fucking dumb to stick a 2015 GTX 900 series card in an i5 from 2011 and not be able to get the best grunt out of the GPU. I'm pretty sure my GTX 670 isn't being bottlenecked, but 2015 series cards is a different matter.
End of the day, it will depend on how my GTX 670 holds up. Fact of the matter is most of these GPU heavy cross gen games are still being made with RAM and CPU limits of current gen systems. Crysis 3 is crazy pretty and makes excellent use of the card, but the scope of game design is still restricted by the current gen. I'm very curious to see how proper next gen games shape up on PC hardware, as I assume a lot more advanced simulations, effects, and detail will become a standard. How those games perform on my GTX 670 will dictate how long I keep it around.