Caffeine has a half-life of five hours in a healthy adult. This means that after five hours after it is consumed, only half of the caffeine remains. A popular energy drink contains 80 mgs of caffeine per each 8.4 ounce serving. You decide to try a single-serving can. 1. Find the number of mg of caffeine in your body for the first five half-lives and the percentage of caffeine in your body for the first five half-lives.