Paraskaragrihyasutra 2.6.1-5

When he has finished the Veda, he should take the bath (by which he becomes a Snâtaka);

Or when (he has gone through) a studentship of forty-eight years;

Or also after (a studentship) of twelve years, according to some (teachers).

(Let him take the bath only) if his Guru has given his permission.

Rules (regarding the performance of sacrifices), (texts) to be used (at the sacrifices according to those rules), and reasoning (on the meaning of the rites and texts): that is the Veda.