Security experts link APT1 to China’s 2nd Bureau of the People’s Liberation Army (PLA) General Staff Department’s (GSD) 3rd Department (Military Cover Designator 61398) The operation have been started in the distant 2006 targeting 141 victims across multiple industries. The Mandiant has also identified more than 3,000 indicators to improve defenses against APT 1 operations and is releasing a specific document that will address them. The longest time period APT has maintained access to a victim’s network was 1,764 days, or four years and ten months.
Source: https://thehackernews.com/2013/02/mandiant-revealed-chinese-apt1-cyber.html