I added 1/2 gal brewed coffee to secondary of a 5 gal batch. Waaaay too much. If you add to an already-fermented beer, go for about a pint. Consider adding lactose to the wort if you'd like to offset some of the coffee bitterness and give it a richer mouthfeel.
If a watched pot never boils, doesn't that imply that watching a boiling pot will cool it down faster?
Note: Reading this post increases your BAC by 50%.