How long did a manslayer have to stay in a city of refuge?

Answer: Until the high priest died

The manslayer had to live in the city of refuge until the death of the high priest who was anointed with the holy oil.

“The assembly is to protect the manslayer from the hand of the avenger of blood. Then the assembly will return him to the city of refuge to which he fled, and he must live there until the death of the high priest, who was anointed with the holy oil.”— Numbers 35:25 (BSB)