Chapter 10 Install and System Administration for FortiOS 5.0 : Basic Administration : General Settings : Password policies
  
Password policies
Password policies, available by going to System > Admin > Settings, enable you to create a password policy that any administrator or user who updates their password, must follow. Using the available options you can define the required length of the password, what it must contain (numbers, upper and lower case, and so on) and an expiry time frame.
The FortiGate unit will warn of any password that is added and does not meet the criteria.