That depends on how much voltage you need to apply? If you're able to OC, with stability, to 3GHz without having to apply extra voltage, and you're also able to keep temperatures the same with an after market cooler, then the cpu should last you as long as it would if ran it at stock.

If you need to apply a small bump in volts, then the life span of the cpu will decrease. However, there's no set in stone rule as to how much less it will live. Extra voltage increases electromigration... basically the transistors wear down... but that's going to happen whether you overclock or not.

If done properly and with care, the life of the cpu is going to last longer than you intend to use it, in most cases. If it's your plan to still be using this chip 10 years from today, then you may want to stay stock. Most guys don't keep chips 10 years.

This is really something that you need to bone up on first though before you jump in. Read everything you can find for the next month. Know everything there is to know about each piece of hardware you own, so you'll have a good idea of what to shoot for and expect before you jump in. That will benefit you tremendously in the long run.