§1. A bishop is obliged to visit the diocese annually either in whole or in part, so that he visits the entire diocese at least every Five years either personally or, if he has been legitimately impeded, through the coadjutor bishop, an auxiliary, vicar general, episcopal vicar, or another presbyter.
§2. A bishop is permitted to choose the clerics he prefers as companions and assistants on a visitation; any contrary privilege or custom is reprobated.
Source: Code of Canon Law (Vatican.va)