At least once a day. There's an old adage among dentists: “Floss only the teeth you want to keep”. If you don't want to lose your teeth, floss every day. Otherwise, you'll be 75% more susceptible to periodontal disease that has been documented to have serious health consequences, e.g. a higher likelihood of heart disease, diabetes, pneumonia and infections. About 45% of American adults have some form of gingivitis, and most adults over 60 have already lost their teeth. Don't be one of them. Floss at least once a day.