Considering the fact that you are a hacker yourself, this shouldnt be so hard for you:
Add a new field to user table which is int(10) then write a code to update it with current unix time whenever user changed his password (or admin changed his, if you want). Write another code to check the time difference between now and in this field whenever user logs (in member.php). If time difference > X, pop up your message or force him to change his password, do anything you like with him..
Regards,
Logician