Charles Ellicott Commentary 2 Chronicles 29:17

"Now they began on the first [day] of the first month to sanctify, and on the eighth day of the month came they to the porch of Jehovah; and they sanctified the house of Jehovah in eight days: and on the sixteenth day of the first month they made an end." — 2 Chronicles 29:17 (ASV)

The time the work took. Beginning on the 1st of Nisan with the purification of the courts, they had cleansed them by the 8th of the month, and came to the porch of the Lord, i.e., to the entry of the holy place. The following eight days were spent in cleansing the two holy chambers, and by the 16th of Nisan the work of purification was done.