There is no set rule for when a violin bow should be replaced or rehaired. Professional players rehair violin bows often, 3 or more times per year, and choose expensive ones that are well balanced and fit their style. A beginner should consider getting a new bow if their own feels not right, is off-balance, and heavy. Getting an opinion from a tutor or comparing it to other bows can help you decide. Also, consider changing a cheap violin bow after some 6 months as they consume faster and might be worth rehairing.