For how many years was Nehemiah governor without eating the governor's food allowance?

Answer: Twelve

From Artaxerxes' twentieth to thirty-second year, twelve years, Nehemiah and his brothers did not eat the governor's food.

“Furthermore, from the day King Artaxerxes appointed me to be their governor in the land of Judah, from his twentieth year until his thirty-second year (twelve years total), neither I nor my brothers ate the food allotted to the governor.”— Nehemiah 5:14 (BSB)