The string material controls a guzheng’s timbre. Modern instruments are usually strung with "Nylon" multi-part strings that have a core of drawn steel, a winding of copper for bass strings, a tight thread layer, then a final nylon winding. The ends are wound in an additional layer of thread and shaped for the anchors. Nylon strings are the most common because:
Relatively cheap to make
Can be made of consistent quality
Durable, last longer
Are more comfortable to play
Have a well-rounded sound
Can be held at a higher tension, producing a louder sound and allowing for faster play styles.
Compare that with the other materials:
Effectively the core of the modern string, drawn wire strings were made of steel, steel wound in copper, straight copper, or brass. Metal strings are more durable than silk, easier to produce consistently, and louder. Their timbre is described as bright, emphasizing higher frequencies that can be unpleasant to the listener. Left-hand ornamentations can be painful to play. Drawn wire strings transmit the nuance of left-handed techniques better than nylon but not as well as silk. Tensions are generally lower, preventing the instrument from being used for modern, faster songs. Sources: Cheng 1991, Han 2013
Silk was considered one of the best materials for strings for a very long time. It transmits nuances that metal and nylon strings simply do not capture. However:
Silk strings are expensive and prone to breaking. They are quieter and require frequent tuning. They are kept at a lower tension in order to emphasize fine nuances, prohibiting modern, fast techniques. Silk strings are said to have a 'dull' tone that does not transmit higher frequencies as well as metal strings. Sources: Cheng 1991, Han 2013
There is a strong sense of culture and history tied in thanks to China's connection to silk but is rarely used today.
While rarely discussed in modern English sources, strings were made from whatever materials people had on hand. The guzheng manufacturer Tianyi published an overview referencing animal hair and animal "tendon" being used as strings:
The zither string has gone through several stages in its history. Early on strings were made of dried deer tendon [species: Cervus elaphus] or hair from the deer's tail. The advantage of these types of strings is their soft, pleasant sound. The disadvantage is their small volume, (易走音?), and their [short] service life.
Carole Pegg writes in The New Grove Dictionary of Music and Musicians that the Mongolian zither the yatga is said to have had strings made from a variety of materials including horse hair and animal intestine (also known as catgut). It is perfectly possible that Chinese zithers were strung with such materials at some point and place in time.
One questions that comes up from time to time is what thickness of string is appropriate for which location on the instrument. This is especially important when people are trying to replace a broken string on an older guzheng with drawn wires. There is no universal standard as to what thickness of string should be used at which point on an instrument, but there are some general trends observable on modern instruments. Below are the thicknesses of strings on 8 different guzheng, ranging from steel to wound copper to nylon. Guzheng A-D are 21-string guzheng with modern nylon strings. The first two are full size, the last two are travel size.The remaining Guzheng E-H are drawn-wire guzheng, strung with steel or copper-wound steel.
All measurements were taken in inches. The second chart has the same measurements converted to millimeters.
The 16-string guzheng used far thinner wires than the 18-string.
The sudden jumps in thickness around strings 14+ are from the addition of copper windings.
Wire is sold by its "gauge" but there are several competing measurement systems. Use this link to convert from diameter to gauge.
Guzheng String Diameters, Inches
Guzheng String Diameters, Millimeters
Modern strings last for different lengths of time depending on how much they are played, how they are tuned, and how energetically they are struck. The brand TianYi suggests in their knowledge base that for someone who practices everyday for 1 hour, strings will last for 2 years. Carol Chang meanwhile suggests that the thinner, higher-pitched strings last 6-12 months, while the thicker bass strings can last more than 2 years.
The real deciding factor is your preference for the sound. Over time the string materials wear out. The timbre of their sound changes. You'll be able to get them to a desired pitch, but the extra qualities that make guzheng music so special will alter. If the sound doesn't bother you there is no need to change strings. I know guzheng players and teachers who only change strings when they break.
Let's get a fear out of the way: yes strings can break but they are unlikely to hurt the player or the instrument. The metal wire in the center of a string is typically what fails first. When it fails all of the string’s tension is passed to the nylon winding which will stretch suddenly. It may startle you but it won't hurt you.
Tension and striking force are what wear strings out. Over tightening strings wear them out faster. If, when tuning, you change the pitch of the strings drastically without lifting the strings up to equalize their tension, that local tension buildup will weaken the string, leading to it failing sooner. Some, especially the treble strings, can break right then.
To avoid these issues, follow the guidelines for tuning and check the charts for bridge placement for each key. If your bridges are close to those locations and you are using the correct string for each space on the guzheng, your strings won't be too tight. If your bridges are way far to the left, towards the tail, that's a sign your tensions will be too high to compensate.
Last thing to consider is that every brand and quality level of string is different. Each set can have a different timbre, and last a different length of time. The lifespans given above are benchmarks. Your mileage may vary. If multiple strings break while playing and it's been a few months since you last bought new strings, consider getting some replacements. If the strngs are new, it's a sign the instrument is at too high of a tension or is being mishandled in some way.