Overclocking is entirely safe. I'd guess CPUs last for about 10 years or more, but CPUs become obsolete within 5 years. Hardcore overclocking/extreme overclocking with extreme voltages will shave a few years off a CPU, but the CPU would become obsolete long before it dies.
Moderate/light overclocking will take reduce its lifespan by negligible amount.
As long as you keep your CPU under the heat limit (monitor it with HW monitor) and don't exceed the stock voltage by a lot (use HW monitor or CPUz), your overclock should be entirely safe and your CPU will last longer than you have a need for it.
Even with stock cooling, you can overclock by a good amount depending on your chip. With an aftermarket cooling, you can overclock a lot more and keep your CPU cool.
I have a E5300 that has a stock of 2.6GHz and I have it overclocked to 3.6GHz with 1.31v and a FSB base clock of 277MHz. Max stock voltage is 1.36v (so it's within stock), while it's safe to push it into the 1.4v range. I have a cheap $12 cooler, and it keeps my CPU under 50'C. The E5300 is safe when kept under 70s'C, and won't throttle/degrade until it gets into the 90s'C.
ie.
1.1v underclocked E5300 = lasts 10 years 4 months
1.2v stock non-OCed 2.6GHz E5300 = lasts 10 years
1.25v 400MHz OC 3.0GHz E5300 = lasts 9 years 8 months
1.3v 800MHz OC 3.4GHz E5300 = lasts 9 years
1.35v 1200MHz OC 3.8GHz E5300 = lasts 8 years 6 month
1.45v 1400MHz OC 4.0GHz E5300 = lasts 7 years
1.55v 2400MHz OC 5.0GHz E5300 liquid cooled = lasts 5 years
Even if you crazy OC it, as long as you keep it within a good temperature range and don't overvolt it by too far, it will last longer than you'll ever need it to.