Detect Button Being Held Down

I want to detect if I have the F Key held down for 0.5 seconds, to call the function PickUp();

var count:float=0.5;

function Update () {
if (Input.GetKeyDown ("f")){
  count-=time.deltaTime;
  if(count<0){
  PickUp();
  count=0.5;
  }
}
else
   count=0.5;
}