Board Thread:League of Legends discussion/@comment-33565980-20171104195051/@comment-3391671-20171124054035

"Out of combat" isn't really clear in general. Thus each champion or item that has a "out of combat" state will define it. Typically, it is when you leave combat, and then the champion/item description will describe how long away from inflicting damage or being damaged (or acting aggressively by casting ability even) you need to be in order to attain "out of combat" status.

The typical seems to be roughly 5 seconds, but again, each champion or item has their own timer.