We are committed to an ongoing process to improve our privacy practices, and have recently taken a closer look at the question of cookie privacy. How long should a web site "remember" cookie information in its logs after a user's visit? And when should a cookie expire on your computer? Cookie privacy is both a server and a client issue.
On the server side, we recently announced that we will anonymize our search server logs — including IP addresses and cookie ID numbers — after 18 months.
Official Google Blog: Cookies: expiring sooner to improve privacy