When is it time for a rehair?

That depends largely on how much you play. Professional players might need quite frequent rehairs. As a student or amateur you might be fine for a year or even longer. It is time for a rehair when you feel the hair looses grip and you want to rosin it more and more often. You shouldn't need a lot of rosin to keep the playing quality consistent. Once every couple of times you play should be enough. If you have rosin dust collect on your instrument and strings every time you play you are using too much. If the hair is fairly new and the ribbon intact and you still loose grip the hair might just be dirty and can be cleaned (I offer this service for free for bows I have rehaired). Another obvious reason for a rehair is a lot of broken hairs. Since bow hair is so sensitive to moisture, tell your bow person before a rehair if you'll be using the bow in a very dry or humid environment, so that they can adjust the length accordingly.