The question I get asked most often as the “stringing guy” is “when should I change my strings?”
Other than the obvious answer - after breaking the existing strings - it's a question that requires some understanding and then making a decision only you can make for your tennis. A little knowledge can really help with that.
An important thing to know about tennis strings is they are constantly stretching and relaxing. Once the machine pulls the end of the string to the set tension, it is clamped off and begins to relax. It's the nature of how strings work, not some weird defect. Different materials lose their tension at different rates, but eventually they all do, and it's always faster right after stringing and gradually slows down. This happens whether you hit a ball or not, although the impact from playing the game generally speeds up the process.
Throughout your tennis journey you will likely play with different racquets and certainly different strings. If you are paying attention, at some point you will decide you like your strings a certain way - they perform well for you and enhance your ability to play the game. They “feel” right. Professional players spend time defining what that means to them so they can have their racquets strung to match that “feel” the day before, or as close to the match as possible. They know they like to play with fresh strings because they've spent many, many hours honing their skills with a racquet strung just the way they like it.
For us mere mortals, the process is usually a bit different. For example, some of us can immediately tell the difference and appreciate when we have fresh strings, because it just seems easier to put the ball where we want to. Others get the fleeting feeling that a newly-strung racquet feels different, but soon forget about it. We might have a good (or even bad!?!?) day or two on the court with new strings, but soon that fades. While we are hitting tennis balls our strings are stretching and then snapping back, although over time they are losing tension. It's known as creep. Remember this process is faster in the beginning and then slows down over time.
There are gadgets and gizmos that can be used to measure how much your strings have relaxed. There are phone apps that measure the frequency of vibrations when you strike the string bed with something firm.
There are expensive machines in specialty shops that can measure how much energy
it takes to stretch the strings a small amount.
There are also handheld devices, both electronic and manual, that give you some indication of how the string tension has changed since the last stringing. They are especially useful if you compare that to the reading from the instant the racquet came off the stringing machine.
To cycle back to the question in the title: why should you care about string creep? The answer: as string tension drops, the racquet plays differently. If the strings are now soft enough that the ball is staying on the string bed for one one-thousandth of a second longer, that can mean the difference between a ball landing inside the baseline or a foot and a half beyond. Fortunately for us, creep doesn't happen like a light switch; instead, it happens relatively slowly, again, faster initially and less obvious over time. Certain types of string also lose their tension at a slower rate than others.
If you are not someone hitting hard enough to break strings regularly, then it is possible your subconscious tennis brain will make adjustments as creep happens without you knowing about it. It will compensate for the changes in tension by changing the way you hit the ball. Few people I know will enjoy missing their targets day in and day out, so something subtle is going on as the strings age. The racquet is now training the player to hit differently in order to keep the ball in play. Fortunately, the changes slow down as the strings truly become “old”.
So, to get past axioms like “change your strings in a year as often as you play in a week”, or “time to change when the tension drops by 20%”, the decision comes down to your commitment level, and possibly your finances. If you appreciate playing with a newly-strung racquet then understand that feeling is finite. Not many recreational players can afford to have new strings every time they play like the professionals do. It's not really practical to think about unless you are stringing your own racquets and have a lot of time on your hands.
It comes down to, then, two ways of approaching strings:
1. You can try to chase a consistent string tension and be willing to make a point of getting them replaced at regular intervals. That will give your practice and play a consistency, so your equipment is always predictable.
2. You can just play with them until they break and accept that you will be playing with strings that change over time and will not perform the same as new.
The only point I would make about the latter option is there are lots of opinions on tennis blogs and articles saying that old strings can cause arm problems. In my stringing practice I have experienced people who come back for a restring when they start feeling pain in their arm. I need to be honest and say I have been unable to find a physiological reason for this to be the case in the research I have done. My gut tells me the strings should get softer as they loosen, so the impact with the ball should actually be less harsh. Assuming anecdotal information reflects reality, it could be that changes in stroke technique that come from playing with loose strings could potentially be the culprit. On the flip side, many people insist on playing with looser strings. That would frustrate me because I would miss the perceived control of firmer strings. Certainly, many advocates of lower tensions experience no issues with their arms or shoulders.
I would be very interested in hearing your thoughts on the matter.