John Gill Commentary 2 Samuel 2:11

"And the time that David was king in Hebron over the house of Judah was seven years and six months." — 2 Samuel 2:11 (ASV)

And the time that David was king in Hebron over the house of
Judah
And over them only,

was seven years and six months ;
to which being added thirty three years he reigned over all Israel in Jerusalem, made forty years and six months; and which, for the roundness of the number, is usually called forty years. See (2 Samuel 5:4) (1 Kings 2:11) .