MovieTexture.duration
var duration: float;
float duration;
duration as 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(); }
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public MovieTexture mov;
    public GUIText gui;
    void Start() {
        gui.text = mov.duration.ToString();
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mov as MovieTexture

	public gui as GUIText

	def Start() as void:
		gui.text = mov.duration.ToString()