I string rackets as a business. It all depends on the player and what type of string they use. Synthetic Gut and Multifilament keep tension much better than the typical poly string. What level player are you and what is your game style?

As a staring point, the rule of thumb is to string as many times a year as you play per week on average. So if you play twice a week, string twice a year. If you are using poly strings, you may need to double it.