Frequency of bow rehairing depends on quantity and style of playing. There are, however, some general guidelines:
Do not harbour the illusion that the hair is still good if it is not torn or otherwise damaged. Playing 3 or more hours a day requires rehairing every 3 months. There is an underlying danger that the structure of the hair is gradually becoming worn out and due to this fact we tend to adapt the technique of playing until we reach the point when we are no longer satisfied with the sound produced. When we have to apply rosin to the hair more often that usual, it is time to rehair. If hair is of good quality, the sound produced by the bow is soft and without noise, and the bow will have a firm but soft grip of strings. On the contrary, the worn out hair will have poor grip which can not be compensated by adding more rosin as this will not only result in more friction but will also produce more noise.