The main juice you'll be squeezing is more electricity, heat, and money. By the time you hit the point where OCing the CPU is how you keep your system feeling fast, you should start looking to upgrade.
Look at the Nehalem chips. They're still going fairly strong and don't need to be OC'd to feel fresh. Considering it's six years old, and should be perfectly viable for another 18 months at least, I think that qualifies as good longevity. My Sandy Bridge machine I built three years ago ( still at stock speeds, ) shows no sign of slowing down, and I can't think of a reason it won't be just as much a gamer in 2017 as it is right now ( with a GPU upgrade, of course. )
Chips and components are lasting longer than they did ten years ago. Barring some huge software paradigm shift, there's no reason a good, strong Haswell i5 won't be more than viable for at least five years, bare minimum. The CPU alone is rarely the bottleneck, it's the mboard and platform as a whole. You don't support newer instruction sets, your memory controller is slow, your bus speed and lanes are limited, etc. Simply speeding up the CPU doesn't offset the rest of the limitations.
Getting five to six years of good gaming out of a platform is perfectly respectable. If you can go seven, you're lucky. Beyond that, and maintaining maximum framerates in games during that time span, simply isn't going to happen unless you've got a lot more money than brains.