Longer than that. I've still got an i5-750 from 2009 and its still competitive. Sandy Bridge (-2500K) and Ivy Bridge (-3570K) were better, but not better by enough to make me want to spend $300+ on a new CPU and motherboard. Haswell (-4xxx?) this year might be the first that's actually worth it for me.
If history is any indication, someone buying an i5-3570K today could probably expect ~3 years before there's an upgrade that's truly worth its cost. Year-to-year processor upgrades are usually incremental, they do not often obliterate anything in that time.
Desktop i7's are quad-cores with hyperthreading, which means they behave something like 8-core processors. They're great, but that's something that benefits a lot of professional video editing/architectural/engineering-type programs, but not so much gaming. An i5 will usually perform approximately the same in games.