Nearly all AT&T cell customers’ call and text records exposed in a massive breach

Corporations are people… except when those people need to go to jail.

It actually seems like it was three months. From a different article

The company wrote that it first learned of the incident in April, but the U.S. Justice Department determined in May and again in June that “a delay in providing public disclosure was warranted” until now.

Either way, we deserve a full answer on that delay

Lesson number 2 should be "you can't do everything for yourself in theory or escape these things in practice, we need sensible regulation of our systems"