Iv Bridge should last you a few years. It all depends on what your expectations are and how much performance you want. If you buy an i5-3570k and within a week you think it is too slow, then your expectations are excessively high.
My current PC is built around a Q9450 @ 3.0GHz. Even though it is over 4 years old, it is still fast enough for my purposes and I do some video encoding which is CPU intensive.
I'm waiting for either Haswell or Broadwell (2014).