How long has the player tapped on the iOS/Android display screen? How can I measure the time?

Question

In my game the player needs to tap on the iOS/Android display screen to make a gameplay action. I want to find out how long the player pressed his finger against the display screen. For example, if the player pressed his finger 1 second on the screen than I could make a different or stronger action when the tap gesture is finished. I thought maybe I could multiply the time of the tap gesture with another value to determine the strength of a gameplay action in my game.

For example: Strength of gameplay action = xx seconds * 1000

But I don´t know how to measure the time that the finger was on the screen. How can I measure the time of the tap gesture?

  TouchPanel.EnabledGestures = GestureType.Tap | GestureType.FreeDrag | GestureType.DragComplete;

  while (TouchPanel.IsGestureAvailable)
  {
      GestureSample gs = TouchPanel.ReadGesture();
      switch (gs.GestureType)
      {
          case GestureType.Tap:
          //  Strength of gameplay action = xx seconds * 1000  ?

          break;
          case GestureType.FreeDrag:

          break;
          case GestureType.DragComplete:

          break;
     }
 }