ControllerColliderHit.moveLength
var moveLength: float;
float moveLength;
moveLength as float
Description

How far the character has travelled until it hit the collider.

Note that this can is different from what you pass to CharacterController.Move, because all movement is constrained by collisions.
	function OnControllerColliderHit(hit : ControllerColliderHit) {
		Debug.Log(hit.moveLength);
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnControllerColliderHit(ControllerColliderHit hit) {
        Debug.Log(hit.moveLength);
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnControllerColliderHit(hit as ControllerColliderHit) as void:
		Debug.Log(hit.moveLength)