http://www.consumergenetics.com/caffeine_test.php. The lethal dose of caffeine is 2 grams of caffeine per day, that is, 80-100 cups of coffee a day.For slow metabolizers of caffeine, the lethal dose of caffeine can be lower.he lethal dose of caffeine is 2 grams of caffeine per day, that is, 80-100 cups of coffee a day.