Animation should be invisible to user
Good animation is invisible. You shouldn’t notice that you’re looking at animation.
— Pasquale D’Silva
Can’t agree more on this. Animation should never distract the user. User should not notice any transition. Having an interface transition too short then user will surprise on the sudden change. Having it too long then user will notice there is an animation there.
In my experience on designing flash motion interface, every transition starts at 0.3s duration before micro tuning each one. I call this Golden Number of UI transition.