How often you restring your racquet will depend on your style of play, the type of strings you use,the type of racquet you use and your frequency of play.
These are some guidelines to help you decide when you should consider a restring.
Racquets should be restrung as many times a year as you play each week. ie: If you play 2 times a week, you need to restring 2 times a year.
Racquets should be restrung after 40 - 50 hours of play.
Nylon strings will lose their tension quicker than others, usually after 30 - 40 hours of play.
When you strike the racquet face against the palm of your hand you should hear a ping. If you hear a dull thud instead, then it is time for a restring.
If you find the sweetspot has disappeared, you need a restring.
If you play a very powerful game and hit the ball hard, you will probably need to have your racquet restrung more often than the average player.
At the very minimum, your racquet should be restrung every 6 months regardless of how often you play. This is because the strings will naturally lose their tension and elasticity over time.
What you thought was a good tension will slowly disappear forcing you to change your game to adapt to the lower tension and gradual loss of control.
Tension is key when restringing. Every racquet will have an optimum tension range detailed on it (eg 55lbs - 65lbs for tennis or 20lbs - 30 lbs for squash etc). If you require more power, choose a lower tension, if you require more control, choose a higher tension.
If you are unsure, start midway and experiment over two or three restrings to find the tension that suits you best.