Without going into any technical stuff, realistically, it should last for quite a long time depending on your needs. At least 2-3 years or so and possibly a lot more.
I remember how about 2-3 years ago people were telling me how I am bottlenecked by my e6300 Conroe, which was overclocked to 2.3GHz. I gave in to all the BS talk and got a 5200 Wolfdale (which was quickly OCd to 3 GHz), and very soon after, a 2500k. I gave the old c2d setup to my dad.
Well guess what. I wasted my money. I recently had to use my fathers computer for some things, and for general gaming and productivity applications, I noticed VERY LITTLE performance difference. Windows, Office, Photoshop, all that stuff was working well and loading reasonably fast. Is it worth spending $190 more to have this stuff load 10-20sec faster? You be the judge. But does it all work just as well once it's loaded? you betcha.
Now the most important stuff. Games...
Again, the dual core e5200 performed wonderfully, and played many resent games very well. (with a good video card off course) And yes, I admit that if it wasn't overclocked it would probably make some games lag, but OCing that chip is so easy, your granma can do it!
So very often the question of buying a new CPU comes down to a great question.
Do you NEED the new CPU or do you WANT one? Are you getting it because you can or because you need to? I bet for at least 70% of people out there, it's "Because they can".
I already keep seeing all the scary news on the internets about how Intel is moving to a new CPU Socket already, and how the newer CPUs integrate better, faster graphics in their chips and all that. supposedly making my 2500K obsolete already. Obsolete for what exactly?
Before getting a new CPU, always run some benchmarks, see if the chip still does what it needs to for your needs, and only if it cannot do that anymore, get a new one.