As you probably already realize your bow string is an essential part of the bow, because lets face it if there is no string then there is no possibility of shooting. Pay attention to the draw-length of your bow because as a string ages it becomes stretched out and this in turn lengthens the draw of your bow causing the draw to become too long. This however does not always mean you need to replace the string, a few twists may bring it back in. You can also prolong the life of a string by waxing it.
Having a spare string is great but when should you replace your string? All you need to do is simply look at the condition of the string. What you are looking for are badly frayed sections, broken strands on the main part, and on the loops. When you see this serving coming undone or badly spreading apart it is definitely time to change the string.