MovieTexture.duration
var duration: float;
Description

The time, in seconds, that the movie takes to play back completely.

Note that duration is only available once the movie has started. Until then duration will return -1. If you need to know the duration of a movie before it is played, consider extracting this information offline and creating a text asset. You can then look up the duration given the movie name.
	var mov: MovieTexture;
	var gui: GUIText;

function Start() { gui.text = mov.duration.ToString(); }