Really depends on the temperature. Overvolting a little wont do anything to it. Buying a stock CPU and getting it say at 1.2vcore aint going to make much difference if you go to 1.25vcore, even though its still best to get it as low as you can.
But, overvolting something to like 1.6 vcore and not providing adequate cooling, will kill it. I cant say how long it would take or anything, but if theres no warnings come up on the motherboard or anything then eventually itll die rather quickly.