The basic rule of thumb for restringing a tennis racket throughout the year is to restring the racket as many times as you play in a week. For example, if you play four times per week, you should restring your racket four times per year. The strings may not show any wear, but putting a fresh set of strings in your racket helps the racket to perform the way the manufacturer intended.
Loss of Tension
Strings lose their tension over time. Some strings, such as multifilament strings, tend to lose tension more quickly. This is because they are made with many fibers, and one characteristic of this type of string construction is that it stretches more. Players who keep their rackets in the heat of their car may also have to restring more often, because the strings lose a significant amount of tension in the heat. A tennis racket left in a car for one hour during the heat of the day loses 18 percent of its tension, according to the famous tennis coach Nick Bollettier. Therefore, if you play with multifilament strings or leave your racket in your car, you will need to restring more often.
Loss of Resiliency
All tennis strings have resiliency or "life." The life of the strings is their ability to return to a normal state after they are elongated. Over time, the strings lose their resiliency and begin to feel dead when the ball is hit, since they no longer have the ability to give energy back to the ball. To keep your racket performing at its best, you may want to restring it once you sense a loss of life. Your play rate can also be a factor. If you have a freshly strung racket but do not hit a ball for 12 months, the strings will lose some of their life.
If you are an avid tournament player, you may want to restring your rackets before each tournament. When you're playing a match in a tournament, you're under specific time limitations. The official rules of tennis limit you to 20 seconds between points and 90 seconds when you change ends of the court. If you break your strings and you don't carry extra rackets, you may be penalized if you leave the court to get a racket and don't return within the time limit. If you do carry more than one racket, make sure all of your rackets are freshly strung. This ensures a uniform feel between rackets if you have to switch.
Style of Play
Hard-hitting tennis players who play aggressively and put a lot of spin on the ball should restring their rackets more often. The strings move back and forth against each other when spin is imparted on the ball. This gnawing action causes the strings to break within hours or days of being strung, and aggressive players will find weekly restringing a necessity. If you hit a relatively flat ball, you shouldn't break your strings as often and may find the rule of thumb for restringing more appropriate.