Normal lifespan of a CPU is around 10 years. When you overclock you can decreas the life of the CPU but some will last 1-2 years overclocked and some will last 10+years overclock. No one can say for sure how long that CPU will last at that speed and voltage. No one can tell you how long it would last at stock speeds. At stock it could die at 4 years or go forever.
I have a Celeron 566 that has beed overclocked to 850 for over 7 years now and still runing. I have been overclocking a long time and I have never lost a CPU.
Most of the time you will replace the CPU before you kill it. But Anandtech has killed a QX9650 with too much FBS Term voltage.
Honestly the biggest issue isn't the CPU dying completely it's unexpected errors. Functional deterioration might occur and you won't even know it until it's too late. Sometimes this means extra BSODs but sometimes it means a lot of data loss or even file system errors.
You might cut the lifespan of a chip by a few years, but if you're interested in overclocking in the first place you will probably be replacing the chip within 2 or 3 years anyway.....
If you're not touching the voltages and keeping it cool the life expectancy won't change. In fact if you overclock and use an aftermarket cooling there's big chance that life expectancy may be increased.
I'd say the cooling solution adds up a lot as well.
I mean, it ain't the same at all having you processor running at 60º at full load than running it at 41ºC at full load (my own experience, lol). The whole case lights up and all share a lot more heat, so the proccessor might hold up a few years, but the other components are not meant to keep up with those temps.
Well, anyway, my old P3@966Mhz (850Mhz PC100 stock) is still up and running, but it has an Athlon XP after market cooler attached to it running at 35º-ish under load and a lot of vent inside the case.
Good cooling is a must, otherwise you'll reduce the lifespan of the CPU and possibly make it unstable. The best solution is an aftermarket cooler like the Arctic Cooling Freezer 7 Pro (or whatever # they're up to now) which blows the heat directly out the back of the case instead of to the side. Put a nice 120mm fan on the back right behind the CPU heat sink and you've got very effective heat removal.
If you're talking graphics cards the situation is a bit different, but again it all comes down to cooling. As long as you can keep temperatures generally the same or better than stock it's no problem, and the best solution again is to blow the hot air directly out the back of the case.