Chapter 1 What’s New for FortiOS 5.2.1 : Authentication : Authentication Blackouts
  
Authentication Blackouts
If five failed logins are made from an IP within one minute, the IP is put on a blackout list. Future logins from this IP are rejected as long as the IP is on this list. The IP remains on the blackout list for auth-blackout-time seconds.
The amount of time an IP is blacklisted can be configured through the CLI:
Syntax
config user setting
set auth-blackout-time 300
end
 
This feature only applies to IP-based authentication schemes.