Limited Fire Rate?

Lately i have been working on a gun script... How can i modify my code, so it has a limited fire rate. Instead of it relying on how fast you can click.

The Code...

 var Bullet : Transform;
 var BulletSpeed : int=300;
 
 function Update () {
     if (Input.GetButtonDown ("Fire1")){    
                 
     var Bullet=Instantiate(Bullet,GameObject.Find("BulletSpawn").transform.position,Quaternion.identity);
     Bullet.rigidbody.AddForce(transform.forward * Time.deltaTime * BulletSpeed);    
     }
 }

• Thanks.