3,975. The Haircuts of a Regular Kohein

Hilchos Bias HaMikdash 1:10

Just as kohanim are only cautioned not to drink wine when entering the Temple, they are likewise only prohibited to grow their hair long when they enter the Temple. This is the case regarding a regular kohein. A Kohein Gadol, however, may never let his hair grow long or tear his garments because he is in the Temple constantly. Regard the Kohein Gadol, Leviticus 21:10 teaches: “He may not let the hair of his head grow long, nor may he tear his garments.”

Hilchos Bias HaMikdash 1:11

Growing one’s hair long means going without a haircut for thirty days like a nazir. Regarding a nazir, Numbers 6:5 teaches: “He shall let the locks of the hair on his head grow long.” Since a nazirite vow is for a minimum of thirty days, we infer that a regular kohein who serves must cut his hair every thirty days.