public TimeSpan Duration {get;}

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

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

{
  var animation = mapView.Map.Animation;
  var duration = animation.Duration;
  if (duration == TimeSpan.Zero || duration <= afterTime)
  {
    // Nothing to scale, leave
  }

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