It's one of the most common questions you'll hear inside of a tennis clubhouse: "Should I get my racquet restrung?"
If you're a casual player who's not breaking strings regularly, it can be difficult to know when it's time for a change. The typical rule of thumb was to change your strings as many times per year as you play per week. So if you play once a week you should change your strings once a year. If you play three times a week you should change them three times per year. 
However, this rule is a little vague and outdated. What if you play a lot but it's a very slow paced doubles match? What if you only play once every few weeks but hit the ball at a higher pace? What if you regularly slam your strings into the top of the net post and claim it's because you're "competitive"?
Not to worry, we're here to help.

Tension

The easiest way to identify if your strings need a change is by their tension. A racquet will begin losing tension immediately, and over time as it decreases, it will have an effect on your shots. 
An easy way to tell if they've lost tension is by tapping the heal of your hand or another racquet against them. Fresh strings will "ping" back at you, while older strings will sound a bit more dead.

Resilience  

Along with tension, a good indicator that your strings may be on their way out is their resilience. 
Loss of resilience will produce more shock in the racquet and reduce comfort. This can also result  in less topspin, and more importantly, can contribute to arm injuries.

The best way to maintain your strings.
via GIPHY

Signs To Watch For

Notches - as your strings develop more wear and tear, they will rub against each other and form small notches. The deeper the notch, the more likely your strings are to break. Half way to 3/4 of the way through your strings and you will likely need to replace them.

Fraying - frayed strings are an obvious indicator. Gut and natural filament strings will fray over time making the visual process easy for you. If they start looking like a Labradoodle then it's probably time for a change. 

Getting wild - missing long and more unforced errors help to let you know when your strings are due for a replacement. It will become harder to control the ball and you might see a reduction of topspin and power. Probably the easiest excuse you'll be able to make for your shoddy play on that particular day.

Rules To Follow

Casual Players - If you're playing tennis once in a while for fun and exercise then you're better off leaving them until they break. It's unlikely you'll notice much difference or will feel any effects of what you're playing with. 

Regular Players - are you a baseliner who hits with heavy pace and lots of topspin? You will probably want to replace your strings more often, if they haven't broken already.
Are you more of a net player who plays recreational doubles or a chip and charge style of play? You likely won't need to string as often.

Competition - Who's your competition? Your strings can change your depth and control, so you want to make sure you're playing with what you're comfortable with. If you're in a high stakes match where the loser buys drinks, then you want to make sure you've got every advantage and can rely on your shots. 
This is why the pros begin every match with a freshly strung racquet, and will often change racquets a couple of times throughout the match. 

Budget - If budget isn't a concern for you, then string often to maintain the best possible performance, but keep in mind everything we've talked about. If budget is a concern, then you will have to adjust accordingly. Inquire with your racquet stringer about the best strings suited to you and your style of play.