Eight hours shall constitute the time of regular service for the members of the police force for such cities during any one day of twenty-four hours; but nothing herein shall be so construed as to prevent the boards of police commissioners in such cities from suspending the operation of such rule fixing the time of regular daily service of such police officers in case of existing emergencies, and said boards shall have full power to determine the existence and duration of such emergencies, and their finding in respect thereto shall not be subject to review by any other power.