If you let your domain name expire, your information will not be immediately erased from the WhoIs database. There is a period of about 90 days when you still have exclusive rights to renew the domain and for this period the WhoIs will still reflect your details.
As soon as the domain name has been released from the Regsitrar and becomes available for registration to the general public, your information will no longer be associated with it.
Some websites that provide WHOIS information, however, keep their own records about domains. Therefore, your personal information will be still available in their databases for some time.