I can't give you a definite answer, and no one else can. But if you're talking about next gen games, and games to come, not that long, is my educated guess.
Assuming 1080p, the 770 is close to maxing out some games, but probably runs better without AA off anyways. The 780 can pretty much max out most games currently, but a few settings -may- need to be turned down.
That being said, future games are going to become increasingly demanding graphics wise. So, if you're only looking at maxing out games, I'd expect a GPU replacement every year. But that's just my take on it. SLI exists as well and will likely max out games a year from now. Hopefully.