Any time you overclock, you are obviously pushing the chip past it's standard specs, but the life depends on a lot of factors (a big one is heat - that's why people get aftermarket coolers and water-cool).
Some people get years upon years with an overclocked CPU.
Most people who push their chips are likely to replace them more often, since they are more of the enthusiast variety.
There is no way to give you a specific lifespan for a chip that is overclocked. If you are going to do it, just make sure you have a good cooler (Xigmatek is a good choice right now).