Before a tournament? Before a big match? Whenever strings break? Never?
Here are some tips and good rules to follow for when you should string your racquet.
If you are a regular string breaker, you should carry a racquet or two depending on how quickly you go through your strings and switch them out. String durability can vary greatly depending on both player and string type. Anywhere from 15-20 minutes of hard hitting by an advanced player, up to 5-10-hours - which is where I see most strings max out in play duration. With that being said, it is important for string breakers to be prepared and plan ahead. Know what you like to play with. Whether that be fresh strings off the stringer, or more worn in string beds. Some players like the feel of fresh strings and will break or cut them out after a certain number of hours of play; regardless of perceived wear. Other players prefer the deadened feel of well worn-in strings. Either way, be prepared, and plan ahead, by stringing ahead. A freshly strung racquet will offer the highest playability and will also be easier on your body as the strings are fresh and elastic.
If you rarely, if ever, break strings - you should make sure that your racquets are in good shape before going into any competition. If you see signs of fraying or significant notching of the strings, I recommend that you get your racquet 1-week prior to competition. This allows you to adjust to the fresh strings and tension prior to competition. If you play regularly through the season, but don't break strings, you should string your racquet annually. For recreational players, racquets will play for a long time... still, to keep them playing fresh, I would recommend re-stringing every 3-5 years.