Pads should be changed every 4-5 hours or every 2-3 hours if the flow is extremely heavy, and if you do not change it frequently enough you can get a yeast infection. Tampons can safely be worn for 8 hours, but if you plan to sleep for longer than that, tampons can become dangerous. Toxic Shock Syndrome is not a myth! Toxic Shock Syndrome is caused by excessive tampon use and can result in fever, vomiting, fainting, and even death. In addition to being safe to wear for longer durations than pads or tampons, the menstrual cup has a much higher capacity than even the most super absorbent tampon out there! A super absorbency tampon can hold 12- 15 ml and can only be worn for 8 hours, while the menstrual cup holds 18+ ml and can be worn for 12 hours safely. The menstrual cup is the obvious winner if you want to stop worrying about bleeding through your clothes and constantly changing your tampon.
FDA; Labeling for Menstrual Tampons; http://www.fda.gov/OHRMS/DOCKETS/98fr/04-19488.htm