Speed Affect Timer

I have a script in c# that allows the user to pick up a speed boost. The script works fine, I can pick up the item, and my characters speed increases appropriately. However, the effect is permanent, and I only want the effect to last for a certain amount of time before defaulting back to 500. Here is the portion I currently have;

 if (other.gameObject.tag == "speed") 
         {
             other.gameObject.SetActive(false);
             AudioSource.PlayClipAtPoint(hit_power_fist, new Vector3(0,0,0));
             speed = 1000;
         }

as for the entire character controller, I have this, incase it helps;

 using UnityEngine;
 using System.Collections;
 [RequireComponent(typeof(AudioSource))]
 public class PlayerController : MonoBehaviour 
 {
     public float speed;
     public GUIText countText;
     public GUIText winText;
     private int count; 
     public AudioClip hit_power_fist;
     void start ()
     {
         count = 0;
         SetCountText();        
         winText.text = "";
         countText.text = "0";    }
     void FixedUpdate()
     {
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
 
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
 
         rigidbody.AddForce(movement * speed * Time.deltaTime);
     }
 
     void OnTriggerEnter (Collider other) 
     {
         if(other.gameObject.tag == "Pickup")
         {
             other.gameObject.SetActive(false);
             AudioSource.PlayClipAtPoint(hit_power_fist, new Vector3(0,0,0));
             count = count + 1;
             SetCountText();
         }
         if (other.gameObject.tag == "speed") 
         {
             other.gameObject.SetActive(false);
             AudioSource.PlayClipAtPoint(hit_power_fist, new Vector3(0,0,0));
             speed = 1000;
         }
     }
     void SetCountText()
     {
         countText.text = "Score: " + count.ToString();
         if (count >= 42) {
             winText.text = "You Win!!";
                 }
 
 
     }
 }