Knowing when to leave the party is a skill. So is knowing when to leave a situation.

Staying too long is its own decision, though it never feels like one. Momentum, politeness and the sense that leaving would be admitting something all keep people in rooms and arrangements well past the point of value. The good exit happens while things are still fine, which is exactly when it feels unnecessary.

Set your exit condition in advance, since you will not be able to judge it from inside. A time, a signal, a thing that if it happened would mean it is over. Then honour it without needing a dramatic reason. Leaving well leaves the door open.