According to the recent U.S. Dietary Guidelines, 400 milligrams of caffeine a day can be part of a healthy diet for most adults. Thatâs two to four eight-ounce cups of most coffees. The Food and Drug Administration says 600 milligrams a day is too much. An eight-ounce cup of Death Wish has 472 milligrams. And for many people a âcupâ of coffee ranges from 12 to 20 ounces.