http://googleblog.blogspot.com/2007/07/cookies-expiring-sooner-to-improve.html

Now, we're asking the question about cookie lifetime: when should a cookie expire on your computer? For background: a cookie is a very small file which gets stored on your computer All search engines and most websites use cookies. Why? Cookies remind us of your preferences from the last time you visited our site. For example, Google uses our so-called "PREF cookie" to remember our users’ basic preferences, such as the fact that a user wants search results in English, no more than 10 results on a given page, or a SafeSearch setting to filter out explicit sexual content. When we originally designed the PREF cookie, we set the expiration far into the future — in 2038, to be exact — because the primary purpose of the cookie was to preserve preferences, not to let them be forgotten. We were mindful of the fact that users can always go to their browsers to change their cookie management settings, e.g. to delete all cookies, delete specific cookies, or accept certain types of cookies (like first-party cookies) but reject others (like third-party cookies).

After listening to feedback from our users and from privacy advocates, we've concluded that it would be a good thing for privacy to significantly shorten the lifetime of our cookies — as long as we could find a way to do so without artificially forcing users to re-enter their basic preferences at arbitrary points in time. And this is why we’re announcing a new cookie policy.

In the coming months, Google will start issuing our users cookies that will be set to auto-expire after 2 years, while auto-renewing the cookies of active users during this time period. In other words, users who do not return to Google will have their cookies auto-expire after 2 years. Regular Google users will have their cookies auto-renew, so that their preferences are not lost. And, as always, all users will still be able to control their cookies at any time via their browsers