Returning visitor stats reliably over longer periods of time in Google Analytics

Hi there. I’m looking into how useful/reliable returning visitors stats in Google Analytics are over longer periods of time, such as two or three years.

It seems that the __utma cookie is set to expire 2 years from set/update, so this suggests that this cookie should be updated each time the user visits the site, and the 2 year expiration reset.

Would I be right in thinking then that it’s reasonable to expect the returning visitors data to be worthwhile over 2/3/4 year periods?