The answer is 0.8 second. A cardiac cycle or also known as a heartbeat approximately takes about 0.8 second to complete one cycle. A heartbeat is fast since it is a muscle that pumps blood throughout the body. The Cardiac Cycle has two phases, first the Diastole phase, wherein the heart ventricles are relaxed, while the heart fills with blood, second is the systole phase, wherein the ventricles contract and pumps the blood out from the heart and takes it to the arteries to be spread all around the body.