Adults and children should change their toothbrush every 3 months because they become worn out and are not as effective as they once were. Exceptions to this would be if you were using an electric toothbrush, and the manufacturer states otherwise. Some electric rechargable toothbrushes have very good brush heads that only need to be changed every 6 months. If you have gum disease, you should change your toothbrush every 4 - 6 weeks because bacteria can harbor in the bristles. You should always rinse your toothbrush out with hot water after every use and change it after you have been sick.
Above article from: dentistry.about.com