§1. The college of consultors must elect a diocesan administrator, namely the one who is to govern the diocese temporarily, within eight days from receiving notice of the vacancy of an episcopal see and without prejudice to the prescript of can. 502, §3.
§2. If a diocesan administrator has not been elected legitimately within the prescribed time for whatever cause, his designation devolves upon the metropolitan, and if the metropolitan church itself is vacant or both the metropolitan and the suffragan churches are vacant, it devolves upon the suffragan bishop senior in promotion.
Source: Code of Canon Law (Vatican.va)