Nazir 1:4-5

Nazir 1:4

If one says that he is a nazir like the hairs on his head, like the dust of the earth or like the sand of the shore, he is a nazir forever and he may shave once every 30 days. Rebbi says that this person may not shave every 30 days (because he has undertaken one continuous nezirus period). One may shave every 30 days if he says “I commit to nezirus like the hairs of my head,” “the dust of the earth” or “the sand of the shore” (suggesting a series of 30-day periods that run back-to-back).

Nazir 1:5

If a person says that he is a nazir enough to fill a house or a basket, they examine him to determine his intention. If he says that he had a large nezirus period in mind, he is a nazir for 30 days. If he says that he committed to an indeterminate nezirus period, then they consider his metaphorical basket as being full of mustard seeds (which are tiny so that it takes quite a lot of them to fill a basket) with the result that he is a nazir for the rest of his life.