how to make it that longer the key is pressed the faster the object rotates?

I would increment some number while the button is pressed and use that for the speed:

 private var rot_speed = 0.0;
 
 function Update(){
     var h : float = Input.GetAxis("Horizontal");
     if(h != 0){
         //$$anonymous$$eep the value positive and increase
         h = $$anonymous$$athf.Abs(h);
         rot_speed += h * Time.deltaTime);
     }
     else{
         //If nothing is pressed, decrease the speed and keep above 0
         rot_speed = $$anonymous$$athf.$$anonymous$$ax(rot_speed - Time.deltaTime, 0.0);
     }
     var rotation : float = rot_speed * Time.deltaTime;
     rigidbody.AddRelativeTorque(...);
 }

This way will only rotate in one direction if you press left or right, but it can be changed to go both directions

thank you so much