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. Partial tolerance to caffeine has been observed in other areas. Studies with mice indicate that after a long period of caffeine exposure, the learning benefits of caffeine observed earlier cannot be found to any significant level. Considering that 80% to 90% of American adults consume caffeine daily, and their mean daily caffeine intake exceeds 200 mg/day, it can be surmised that a large fraction of the U.S. adult population is completely tolerant to most of the effects of caffeine.
Seven days? Wow.