How long in your opinion do you expect the GTX 760 to last?
I'd expect at least a couple of years. But it depends on a lot of variables.
For example, until I built my first rig last December, I was still gaming on a 9500 GT.
Depends on what you mean by viable. Playing future games at ultra-HD at max? Not going to happen. Able to push decent rates at 1080p for future games while compromising settings? SEveral years unless we see a large boom in technology. Besides that, most of the games that people are playing don't really require all of the horsepower available. Benchmark games aren't usually the ones that people plunge tons of hours into (aside from modded skyrim).
I think it will last until Pascal GPUs are released. Two 760s still destroy any single high end card (except maybe the Titan Z) performance wise atm, so it's great if you can't wait to game and you know you'll have another $250 soon.