You should be brushing at least twice per day and flossing at least once per day.
Brushing and flossing help control the plaque and bacteria that causes dental disease. Plaque is a mixture of food debris, bacteria, and saliva that sticks to the teeth and gums. The bacteria in plaque convert certain food particles into acids that cause tooth decay. Also, if plaque is not removed, it turns into calculus (tartar). If plaque and calculus are not removed, they begin to destroy the gums and bone, causing periodontal (gum) disease.
Plaque formation and growth is continuous and can only be controlled by regular brushing, flossing, and the use of other dental aids.