§1. Young men who intend to enter the priesthood are to be provided with a suitable spiritual formation and prepared for their proper duties in a major seminary throughout the entire time of formation or, if in the judgment of the diocesan bishop circumstances demand it, for at least four years.
§2. The diocesan bishop is to entrust those who legitimately reside outside a seminary to a devout and suitable priest who is to be watchful that they are carefully formed in the spiritual life and in discipline.
Source: Code of Canon Law (Vatican.va)