Caffeine is typically absorbed through cellular membranes within 45 minutes after initial consumption. From there, it takes the average adult 4 to 6 hours for half of the caffeine consumed to be metabolized. So if you drink 2 cups (about 200 milligrams) of coffee at 7 am, by lunchtime, 100 milligrams will have been metabolized and digested. By 4 or 5 pm, another 50 mg will have been digested, and by about 8 pm, only 25 mg of the original 200 will still be remaining.