Information
The PASS_MIN_DAYS parameter in /etc/login.defs allows an administrator to prevent users from changing their password until a minimum number of days have passed since the last time the user changed their password. It is recommended that PASS_MIN_DAYS parameter be set to 7 or more days.
Solution
Set the PASS_MIN_DAYS parameter to 7 in /etc/login.defs : PASS_MIN_DAYS 7
Modify user parameters for all users with a password set to match:
# chage --mindays 7 <user>
Notes: You can also check this setting in /etc/shadow directly. The 4th field should be 7 or more for all users with a password.