If you can see through it, it isn't down. When it is blacker than coal miner's lungs, it is done. Take the bag out. You want that sugar rush to go along with the massive caffeine jolt you are about receive. However, what you MUST not do, under any circumstances, is add milk.
Milk is the antithesis of caffeine.
They do not go together. Enjoy milk on its own. Its good for you. It gives you healthy bones. However, it will not wire you for sound.