You should never wait until she is fading like this. The first sign that she is not putting effort towards you turn your back and walk away. Unfortunately what most men do is that they become more clingy and start trying harder... all this does is reinforces in the female mind that she is better than you, which turns her off. This is why being a challenge to women works... they are genetically disposed to pursue and want men that are better than they are... you put a woman on a pedestal and act like a pathetic butler and she starts to wonder why she's with you.
Plenty of other women in the world, start fresh with a new one.