Thanks to Hammer Nutrition client Dr. Michael Cohen for this information:
Tolerance does develop with chronic caffeine use, even if it is only 1-2 cups per day. Caffeine stimulates adenosine receptors in your nervous system and with chronic use your body creates more of these receptors,creating the tolerance. This is called up-regulation (more of the receptors need to be stimulated for the desired effects which results in more caffeine needed). True that the half-life of caffeine is 6 hours, which means it takes 30 hours - 5 half-lives - to be completely gone from your body; however the receptors take a bit longer to downregulate. This can take anywhere from 1-5 days, differing per individual and based on the amount of routine caffeine intake. I routinely recommend people refrain from caffeine for 5 days before races for the maximal effects of caffeine if you plan to use it during your race.