Ammonium chloride will be utilized almost instantly by corals, clams, macro algae, etc...The key is dosing a small amount throughout the day. You can dilute it so you don't have to worry about overdosing when using a dosing pump, but I dose every time I feed the tank, which is 6-8 times a day. You'll have to monitor alk while you're dosing it, as it will add to its consumption. Remember, nitrate is the end process and doesn't actually relate to nitrogen, it just means that you have a surplus. Corals struggle to utilize N & P directly from the water, so they get it through other means much easier and quicker.
I also think that if most people would just feed their fish more, you would get the same results. I dose ammonium chloride because most of my tanks have been nitrogen limited, mainly because I don't keep a good density of fish, which I'm trying to slowly change.