Duration

Duration is how long motion lasts, and it has to answer to frequency. Keyboard-initiated actions and anything a person does 100+ times a day get no animation at all. Actions seen tens of times a day get reduced motion. Occasional interactions get the standard budget, and rare or first-time moments are the only place delight belongs. For actual interface elements, stay under 300ms; anything slower needs a reason. A single duration applied to every transition in a product is the tell that nobody chose the numbers.