The rate of movement must also match. Editors often want to end a shot as an object leaves the frame and cut to the next shot as the object enters, such as when a person walks out of one room and into another or a car roars out of a garage and down the street. In this case, cut the first shot as the object passes beyound the edge of the frame to the second shot about six frames before the object enters the frame. It takes about a fifth of a second for viewers' eyes to switch from one side of the frame to the other. During this time the viewers' eyes are un-focused, and whatever is taking place on the screen becomes scrambled and the transition is less visible.
Viewers lose interest in a shot very quickly once they understand the basic visual information.
You have to consider several things in deciding shot length. First, is the information you want to convey. The simpler or more familiar the object, the shorter the shot. A two second LS of the Statue of Liberty is enough for most viewers to recognize what they see. However a 20 second LS of The Forbidden City in China might not be enough.