You build up a tolerance to caffeine very quickly. Much more quickly than I think most people realize. From the Wikipedia article on caffeine(which cites origin

You build up a tolerance to caffeine very quickly. Much more quickly than I think most people realize. From the Wikipedia article on caffeine(which cites original research): Caffeine tolerance develops very quickly, especially among heavy coffee and energy drink consumers. Complete tolerance to sleep disruption effects of caffeine develops after consuming 400 mg of caffeine 3 times a day for 7 days. Complete tolerance to subjective effects of caffeine was observed to develop after consuming 300 mg 3 times per day for 18 days, and possibly even earlier. In another experiment, complete tolerance of caffeine was observed when the subject consumed 750–1200 mg per day while incomplete tolerance to caffeine has been observed in those that consume more average doses of caffeine. For reference, a grande drip coffee from Starbucks has about 330mg caffeine. and on withdrawal: Withdrawal symptoms — possibly including headache, irritability, an inability to concentrate, drowsiness, insomnia and pain in the stomach, upper body, and joints — may appear within 12 to 24 hours after discontinuation of caffeine intake, peak at roughly 48 hours, and usually last from one to five days, representing the time required for the number of adenosine receptors in the brain to revert to "normal" levels, uninfluenced by caffeine consumption.