You might need a rehair if the bow hair becomes dirty, or when you break enough hairs on the bow. If you rub the hair of the bow with your fingers, the natural oils from your skin will ruin the bow hair. Sometimes near the frog, someone's bow hair might be brown instead of white, this is why. When this happens, you should take your bow to your luthier so he can clean or change the hair. It is normal to sometimes break bow hairs, but if you are constantly breaking them every time you play, it means that there is a problem with the way you are drawing the bow, and you should ask your teacher for advice. At any rate, if you end up with too few hairs on your bow, you should get it rehaired.