Teach me about half life and decay.

So the half life of 200m of caffeine in a human male is about 6 hours.
Does this mean in 12 hours all of the caffeine all of the caffeine is out of a humans system or does it decay at a slower rate the smaller it gets?
How long does it actually take to remove all the caffeine if so?
Is there a difference between pre and post half life caffeine(or anything for that matter)?
What are the formulas for finding these answers?