Albert Barnes Commentary 1 Chronicles 9:25

"And their brethren, in their villages, were to come in every seven days from time to time to be with them:" — 1 Chronicles 9:25 (ASV)

See the marginal references. If the number of gatekeepers was 172, as stated in Nehemiah 11:19 (that is, 168 in addition to the four chief gatekeepers), and the number on duty at any one time was twenty-four, as under David in 1 Chronicles 26:17–18, then the turn for the divisions to stand guard came every seven weeks.