It takes months for most computer intrusion victims to learn they were breached. Unfortunately, the hackers get busy much sooner, often stealing data within days if not minutes.
Once inside the network, the bad guys first might install their favorite malware, including keystroke loggers to gather passwords (yes, even those strong passwords you worked so hard to create); turn off security notification alerts; disable anti-virus programs and firewalls; clear log files of their activity; lock legitimate users out of their accounts; create new user accounts for themselves so they can sign in like decent folks; and, basically do whatever else they like, whenever they like, from wherever they like.