AnimationState.length  

// Print the length of the walk animation in seconds
print (animation["Walk"].length);

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
print(animation["Walk"].length);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
print(animation['Walk'].length)