used to be every 2 - 3 years, but seeing less need now. My Macbok Air is approaching 3 years and is plenty fast, and the Macbook Pro is 4 years old I think (Sandy Ridge quad core i7) and plenty fast.

I might get a new one one of these days, probably replace the Macbook Air with something only to get a better screen but it's 11" and it's really an incredible machine other than battery life.

I think the improvements that are coming are more in the realm of battery life, displays and storage rather than CPU speeds which are plenty fast enough.