When do we need to change our toothbrush?

You can imagine a new toothbrush which you just purchased from a supermarket and the bristle is very nice, very even.
But when you use it for a while, again as you subject to the pressure you applied on the toothbrush so when you see the bristle of the head, and sort of go all over the place, it's very flattened, and if you use a very old toothbrush, with the head, you know is all over the place, you can potentially damage the gum and the teeth and then it's probably a time to change.
As I said, generally they last for three months.