According to the American Dental Association, a person's teeth should be brushed at least twice a day. It's a good practice to brush once in the morning and again before going to bed at night.
Brushing after every meal and snack is best, but adhering to this brushing schedule is not convenient for most people. Brushing the teeth removes food and plaque, which is the sticky film that develops on teeth. Plaque contains bacteria, which releases harmful acids. The acids on teeth can weaken tooth enamel and cause cavities. These acids can also cause plaque to harden into tartar, which is even more difficult to remove.