n: a Zen Buddhist riddle used to focus the mind during meditation and to develop intuitive thinking
Q: How long should you stay at something?
A: However long it takes to get what you came for.
Q: How do you decide what you came for?
A: You don't, you discover it.
Q: How do you discover it?
A: You notice what isn't there anymore when you feel like leaving.
From "Refuse to Choose" by Barbara Sher.