As I wanted to try for myself as well I came up with a simple form of it. The object in this case is only going up and falling down at a rate that is not the gravity. You still need to perfect it but at least you got a lead. What would you learn if I just give you the answer?

var Start:float;    
var End:float;
var force:float = 0.0f;

function Update(){
      // when space is pressed down the counter starts
      if (Input.GetKeyDown (KeyCode.Space)){
         Start = Time.time;
    }
    // When space is released the counter stops note that GetKeyDown/GetKeyUp does not do anything while keeping pressed
    if (Input.GetKeyUp (KeyCode.Space)){
         End = Time.time;
         force = (End-Start)*20;  // here you compare the 2 values I had to multiply by 20 to get it up (that's what she says) but you might have to alter this eqution to get what you want   
    }
    // the result is applied to the object rigidbody, only up in this case here again yo need to work it out
    rigidbody.AddForce(Vector3.up*force);
    force = force*0.9;  // force is decreased in time to get the object falling down
    Debug.Log("Start: "+Start+" End: "+End+" Force: "+force);//Just for debug...
}

Hope that helps.