Duration of animation

Finally I decided to create my dribbble account. I want to start it by posting few shots about main rules in the animation of user interfaces. There are a lot of sources where they are described, but I have a plan to collect all of them in one source and to publish a full article soon! And here is my first shot about duration of animation. When elements change their position or state, the duration of the animation should be slow enough for user to understand that the change has occurred, but also fast enough not to keep user waiting for it.
So stay tuned, new animation rules coming soon!