How long did Zimri reign as king of Israel in Tirzah?

Answer: Seven days

In the twenty-seventh year of Asa of Judah, Zimri reigned in Tirzah for only seven days.

“In the twenty-seventh year of Asa’s reign over Judah, Zimri reigned in Tirzah for seven days. Now the troops were encamped against Gibbethon of the Philistines,”— 1 Kings 16:15 (BSB)