How do I destroy a gameobject after 5 seconds

I have this shoot script:

 var projectile : Rigidbody;
 
 var speed = 20;
 
 function Update() 
 
 {
 
     if( Input.GetButton( "Fire1" ) ) {
 
         var instantiatedProjectile : Rigidbody = Instantiate(
 
 projectile, transform.position, transform.rotation );
 
  
 
         instantiatedProjectile.velocity =
 
 transform.TransformDirection( Vector3( 0, 0, speed ) );
 
         
 
         Physics.IgnoreCollision( instantiatedProjectile. collider,
 
 transform.root.collider );
 
     }
 
 }

And I want the bullets to go away after 5 seconds.

Thanks