From Meta Stack Exchange:
Accounts which are deemed to be abandoned will be deleted, and the user information on their questions and answers will be denormalized into text fields.
Cookie-based unregistered accounts will be deleted when:
The user has not visited the site in six months
AND
The user has less than 50 reputation.
OpenID registered accounts will be deleted when:
- The user has not visited the site in six months
AND
- The user has only 1 reputation (i.e. has never received an upvote)