§1. The function of a diocesan administrator ceases when the new bishop has taken possession of the diocese.
§2. The removal of a diocesan administrator is reserved to the Holy See. If an administrator resigns, the resignation must be presented in authentic form to the college competent to elect, but it does not need acceptance. If a diocesan administrator has been removed, resigns, or dies, another diocesan administrator is to be elected according to the norm of can. 421.
Source: Code of Canon Law (Vatican.va)