How long did David reign in Hebron over Judah?

Answer: Seven years and six months

David reigned over Judah in Hebron seven years and six months, then thirty-three years in Jerusalem over all Israel and Judah.

“And the length of time that David was king in Hebron over the house of Judah was seven years and six months. In Hebron he reigned over Judah seven years and six months, and in Jerusalem he reigned thirty-three years over all Israel and Judah.”— 2 Samuel 2:11, 5:5 (BSB)