From [Meta Stack Exchange][1]:

> 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:
> 
> 1. The user has not visited the site in six months
> 
>    **AND**
> 
> 2. The user has less than 50 reputation.
> 
> OpenID registered accounts will be deleted when:
> 
> 1. The user has not visited the site in six months
> 
>   **AND**
> 
> 2. The user has only 1 reputation (i.e. has never received an upvote)


  [1]: https://meta.stackexchange.com/q/48262/270345