Dropdead777 :
3-4 years..Personaly I'd go with 3570K as it is "safer", easier and more "boostable" to overclocked and I'd go to an upgrade after 4 years as 3570K I believe keep you up in gaming for quite some time. 4670k runs hotter and not much oc potential.
Sorry ,lol, I had my Pentium D for about 8 years now and just now I feel the need to upgrade,(just build a new pc) so the Haswell cpu should last you for a good 10 years if you use it for gaming, you probably shouldn't have the need to upgrade cpu for a long time, but like I'm saying, depends what is your current cpu, if you are like me coming from an older system then it is worth it, even if you are too picky with stuff like me, you can upgrade it in 3-4 years but I think this should last longer and current games technology will last for much longer if you know what I mean.
I hope somehow this helps you, I am looking at building a similar PC as you are.