Many string players, especially beginners, are in the dark when it comes to understanding how their bow works, in particular, when to have it rehaired. This is a quick primer on when and why to rehair your bow regularly.
The hair on your bow is most likely horse tail hair, more precisely Mongolian stallion tail hair. Although some vegan alternatives are now available (and seem to perform relatively well) horse hair is still considered the gold standard for bow hair.
The horse hair on your bow will stretch over time and use. Loosening your bow after each use will prolong the life of your hair by not keeping it in a stretched state when you are not using it.
The purpose of the frog is to stretch the hair tight; when you tighten the button at the end of the stick, you are pulling the frog towards the end of the stick, tightening the hair. The frog only has about a 1/2" length of travel, then it hits the end of the slot that it slides in, and can no longer move. When that happens your hair is as tight as it is going to get, and turning that screw harder, or putting a pair of pliers on it, is just likely to break your button, screw, or even crack your stick.
To prolong the life of your hair, and get the best sound out of it, avoid touching the hair with your fingers or hand. Oil from your hands will create 'dead' spots on the hair. If you start to have hairs breaking at either end of the stick, this is a sign your bow and/or case may be infested with bow bugs, also known as museum beetles. Empty your case out and put it outside, opened up, in the sunlight for a couple days to chase the bugs out. It's best to keep your bow out of your case in some sort of light when not in use to discourage bow bugs from eating your hair.
If your bow becomes over-rosined, you can wipe off the excess rosin using a clean, dry cloth. You should also wipe the rosin off your stick after each playing session to keep it from adhering to the varnish on the stick. Use a soft, dry, clean cloth for that also.
Here are the main guidelines for when to rehair your bow:
~At least once a year (more often if you play a LOT or really hard. Some pros bring us their bows every 2-3 months!)
~When you can't tighten the screw any more
~You break a lot of hairs when you play, especially on the playing edge
~One of your plugs* slips, and your hair comes loose
~You rosin it and rosin it and it still seems like it needs more rosin
~Your spread wedge** comes out
*The hair is held in place in the tip and the frog end of the stick via small, wooden plugs.
**the spread wedge is a small piece of wood beneath the hair at the end of the frog. The ferrule holds it all in place. The purpose of the spread wedge is to spread the hair out evenly the width of the ferrule. Look at your bow and you can see what I'm talking about.