It depends on how much voltage you put through it. You can overclock it without upping the voltage which shouldn't hurt the CPU much at all.
They are meant to last 10 years at 100%.
Lets say you move the voltage from 1.4 to 1.6, you may only get 3-6 years @ 100% of pure life. Most likely you won't have your computer running full load all the time so it should still last much longer.