It could actually last you a good 5-6 years, actually. The games industry isn't exactly driving newer graphics engines that will even pull a single GTX 680 at 1920 x 1080 resolution to its knees for a good long while. They end up driving away potential customers if they need a super gaming computer in order to play it.
Unless there's something really incredible with the next iteration of DirectX 12, most games will usually use OpenGL or stick with DirectX11 for the next 3-4 years.
In my opinion, Nvidia isn't going to make graphics cards that will go crazy on power and drive FPS that much higher than what a GTX 680 gives out now. They're focusing on less power consumption (thus more voltage locking), and thus less heat.
AMD, however, is still going to keep upping the ante in FPS, increasing GPGPU compute, allowing voltage unlocking, and to hell with heat (though at stock voltages, will be quite efficient).
I'm saying it now. AMD will be the cards for the enthusiast market. That you can find HD 7970's for ~$300 when overclocked beat out overclocked GTX 680's that are around at least $100 more expensive is a sure sign of that happening now.