this is interesting:
http://en.wikipedia.org/wiki/Caffeine#Metabolism

so 3-4, maybe longer? I wonder if I drink enough that my liver doesn't can't process caffeine that fast?

Maybe if you started making your evening coffee weaker? Coffee is generally made too strong as it is. Get some good coffee for the evening. I make mine around the consistency of tea, instead of the consistency of syrup like many restaurants and my old roommate.