How often you restring will depend on a number of factors: your racquet, style of play, string type, surface and frequency of play. Generally it is recommended that you restring after 40 - 50 hours of play. If you're not logging your hours of play, the general rule of thumb is that your racquet should be strung as many times per year as you play in a typical week. As a minimum you should restring every six months because most strings will go dead in this time regardless of the frequency of play.
Waiting until strings break to replace them is not an ideal approach. As the ball makes contact with your strings, a violent collision takes place. During this collision the strings absorb and release a tremendous amount of energy. The good thing is, like a fresh rubber band, new strings absorb the impact and spring back into shape quickly. This ability to absorb impact and return to form is known as "resiliency." As strings age, their properties begin to decompose and weaken. Specifically their resiliency decreases meaning they are no longer absorbing the impact or propelling the ball as they did when new. As a result your arm absorbs more impact and you have to work harder to generate pace on your shots. This can lead to arm, elbow or shoulder fatigue/pain. As soon as you notice the playability of your racquet beginning to fade, it is most definitely time to change strings.
At Guts and Glory Tennis we recommend restringing at the beginning of each new ALTA season. Starting the season with a fresh set of strings will allow the majority of players to make it through without having to worry about restringing. Some chronic string breakers, or those playing in multiple leagues, may not make it the entire season without breaking a string. If you are fortunate enough to make it to post-season play, (the playoffs!), we recommend that you restring early in the week, hit with your racquet in a practice session before the match and go into the match with fresh strings. You want to be playing your best in big matches and equipment can play an important role. There is nothing quite like playing with a freshly strung racquet. Many pros change racquets several times per match. They do this so that they are constantly playing with fresh strings with the liveliest playing characteristics.