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Rav Masna and Bar Pada disagree about the length of a Nezirus with no specified duration, mid'Oraisa. Rav Masna maintains that the Nezirus is 30 days long, and Bar Pada maintains that it is 29 days long. Bar Pada proves his view that the duration of Nezirus is 29 days from the statement of the Tana, Rebbi Eliezer, who says that a Nazir becomes Tamei on the thirtieth day of his Nezirus, his Tum'ah is only "Soser Shiv'ah" -- he does not observe another 30 days, but rather he waits only seven days, after which time he may bring his Korbanos and conclude his Nezirus. The reason why he does not have to observe another period of Nezirus is that his Nezirus already ended before the arrival of the thirtieth day. This proves that Nezirus lasts for only 29 days.
We explore the commentators further on the laws of nezirut