It is usually recommended to change your toothbrush every 6 months and use a new toothbrush because the bristles of the toothbrush will eventually fall out. Patients who have had orthodontics will generally find that the bristles on their toothbrush tend to dry out and wear out sooner than usual.
This is very normal because braces are harder than regular teeth and toothbrushes tend to work very hard to clean teeth, so an orthodontic brush can be replaced every 4 months.