If you are using a regular toothbrush, you should change it every 3 months. However, if you are using an electronic toothbrush, replace the brush heads every 6 months. The bristles become worn out after a certain period of time and are not as effective as the first few months when you just got them.
If you are suffering from gum disease, change your toothbrush every 4-6 weeks because bacteria tend to live in the bristles. It is also important to rinse your toothbrush with hot water after each use or to toss it after getting sick.