If you don't mind using Tweens, I'd recommend TweenLite for this one.
//Add function called in 5 secs
TweenLite.delayedCall(5, handleTimeUp)
private function handleTimeUp():void
{
TweenLite.to(reset_balloon, 2, {alpha:0});
}
And if for some reason you want to kill pending delay call (for example on mouse click);
reset_balloon.addEventListener(MouseEvent.CLICK, fadeBalloon);
private function fadeBalloon(event:MouseEvent):void
{
TweenLite.killDelayedCallsTo(handleTimeUp);
}