The length of time an animation takes from start to finish, typically measured in milliseconds.
Keep hover feedback under 150ms. At 300ms the interface feels like it’s lagging behind the cursor.
Duration should scale with distance and size: small elements moving short distances want 100 to 200ms, full-screen transitions can hold 300 to 400ms. Past half a second, motion stops feeling responsive and starts feeling like waiting. When in doubt, shorter.