Forty years - In all forty years and six months. See 2 Samuel 5:5, and 1 Chronicles 3:4. The Jewish writers almost universally omit the fractions of a year.
Related Commentaries of 1 Kings 2:11
1 Kings 2:11
11 And the days that David reigned over Israel were forty years: seven years reigned he in Hebron, and thirty and three years reigned he in Jerusalem.