u can use ur brush until it is bended or worked out . usually it lasts 3 to 6 months

The brush is not a mere brush, but an essential part of the  cleaning syatem. Depending on the brushing habits , the brush will last . Brush should not be changed frequently , only if the need be. With the correct way of brushing,  the brush will definitely last for long periods of time.

Consider changing the tooth brush when the bristles get frayed as the frayed bristles donot reach out to all surfaces of your teeth that ensure proper cleaning. 3 months is generally a good period to consider changing the brush. Generally those who have kids with habit of chewing the brush while brushing, we recommend them to change often.

Replace toothbrushes every 3 to 4 months. Consider getting a new toothbrush sooner when in doubt, look at the bristles. If they are frayed, they won't clean teeth as thoroughly then you may need to replace more often.