Four years. There are different requirements for the promotion of police rank: police officers and superintendents are promoted to one rank every three years, but it takes four years for a first-class superintendent to be promoted to a third-class superintendent. The training time of police in colleges and universities is included in the promotion period of police rank. Prison police refers to the police who are engaged in prison management according to law, referred to as prison guards. Need to pass the civil service examination, belonging to the scope of civil servants, mainly responsible for the management of prisons, juvenile detention centers, drug rehabilitation centers and other units.