There is no answer, but if you have to ask then you shouldn't do it.
A lot of people say that but I don't really get why. If you're going into overclocking you would want to get a vague idea on what happens.
A couple here have said it depends purely on heat, but I'm pretty sure, even running the cpu at 0C wouldnt extend the life if you were running it at 1.65v.
I found a detailed thread about it somewhere on another forum - I'll try and find that but I'm pretty sure it concluded:
Due to electromigration, voltage shortens a processers life more than Heat which shortens a processers life more than frequency.
Maybe in the coming years, someone should try and construct a database of how long cpus take to die, or how often, if they have been overclocked carefully (that doesn't mean putting high voltages thorough the cpu, it means not making mistakes.)
I'm not sure I've read of any, 'my cpu just randomly died', with the answer being 'thats the risk of overclocking'.
Its always a psu malfunction, or dodgy thermal paste. ..or something
A lot of what Ive said is probably misguided. . .but my 2 cent anyway