The answer is that it depends on the lenght of the film and the bitrate that the film was encoded at. Bitrate is the number of bits (as in data) per second are used to store the film. Films that are cartoons, for example, require considerably fewer bits per second to represent the video content, as do films that have many scenes where much of the scene is unmoving or low in detail.
If you purchase a 1080p video from the iTunes store, it will take about 2.6 G per hour, on average. Some will be a little more, and some substantially less. You can also purchase lower resolution formats that require less space.
If, however, you are using your own content, the size can be far more variable. Newer iOS devices could handle bitrates that would produce movies 10x as large and still work. The higher bitrate video will look better, to a point, but at the cost of space. Apple settles on 2.6 G as a practical size so as to fit iOS devices and not take up large amounts of storage on either the devices or the computers with which they sync (and also to decrease the cost of delivery).
For what it is worth, DVD-quality video is typically in the range of 400 - 500 MB per hour.