Most commonly hairs break from the playing side edge. Continuing to play on a bow that has many hairs missing from one side can not only cause the bow to respond less effectively but can bring on or aggravate warping of the stick. Should you find yourself using more rosin than usual and not getting enough grip on the strings, it is time to change the hair. As a rule of thumb, most players find they need a rehair at least once a year. It is important to have your bow rehaired when the hair is too long or short.
Hair length can change drastically with the change of seasons and humidity levels.
Ignoring a bow in this condition can lead to damage of the inner working parts of the bow, specifically the eyelet, and may cause cracks in the butt of the bowstick. It's comfortable the way it is worn down. The leather thumb grip and the metal, plastic or whalebone winding beneath it play an important role in protecting the bow stick from the abuse of playing. Depending on your bow hold and body chemistry, holding the bare wood of your bow can cause the wood to be worn away, thus weakening the stick and affecting the bow's value. There is a piece of my bow tip plate missing.