public void ScaleDuration( 
   double factor
)

public void SetAnimationLength(TimeSpan length)
{
  var mapView = MapView.Active;
  if (mapView != null)
    return;

  var animation = mapView.Map.Animation;
  var duration = animation.Duration;
  if (duration == TimeSpan.Zero)
    return;

  var factor = length.TotalSeconds / duration.TotalSeconds;
  animation.ScaleDuration(factor);
}