Ideally, you should change your tampon before the bacteria has a chance to colonise, Dr. White says. "The more frequently you change them, the fresher they are, and the less likely the bacteria will grow," she says. You should always follow the instructions that come with your tampons, which usually state that you can leave it in for four to eight hours , although Dr. White says that eight hours is really pushing it. And if you're sleeping more than eight hours, then you should just use a pad so you don't have to worry about changing it at night, she says.