What processor do you have ? what's the initial speed ? What speed you want to OC it to ?What temperature are you running it OCed ?
Yes running your product beyond its designed specs will shorten its life - how fast depends on many facotrs - in most cases the effect is long term and most people will have upgraded long before it dies on you - as it runs hotter, and overheating in the long run is not good for your processor - One thing to keep in mind, get a good aftermarket cooler and use quality thermal compound to keep your CPU as cool as possible during your OC to avoid running it close to its threshold or above it all the time. I can't imagine me staying with the same CPU for mroe than 2 years, as I often upgrade so I don't think there is much to worry about - just play it safe and always overclock little by little at a time, don't use huge increments at once - take little at a time and test the stability of your CPU and keep an eye on the temps, keep a comfortable gap between the CPU's maximum temp and your running temp. Eventhough you can safely run at close to top temps, if it's 24/7 it does take some life away from your CPU, and can even shorten it by half in some cases. but unless you plan on upgading in 5 years I wouldn't worry, not i'm not condoning or telling you you should overclock, but if you get rid of the stock cooler and use a good quality one with a good compound and run your CPU at reasonable temps, then you are good for many years - now when your CPU is idle make use of the power saving features, like cool and quiet for AMD processors for example, if you take safety precautions I don't see why you would damage your processor - the cooler you run it the better - the only thing that will damage it is improper thermal transfer, overheating (as with any other electronic component in your system), and extreme overclocking without proper cooling.
If you read the forums here and elsewhere you will see that some people are running OCed system successfuly and have been doing so for quite some time.