as you grow up, you learn a lot of things both at school and at home.
the adults around you, i mean, the teachers and the parents teach and tell you a lot of things.
but how many of us were told exactly when to stop using your teethbrush, throw it away and start to use a new one.
i am sure it depends on each person the timing to change his/her teethbrush.
by the way i usually change mine before the brush gets curly.