Guzheng strings are commonly steel core wound with nylon, though plain steel and other types exist. Different gauges differ in tension, tone and feel. When replacing, change the whole set — mixing old and new strings produces inconsistent tone and response.
Strings gradually lose elasticity with use, and the outer winding can loosen through prolonged friction. When a string develops a pronounced sandy tone, produces more noise when pressed, or will not hold pitch despite repeated tuning, it is usually time to replace it.
There is no universal replacement interval: it depends on how much you play, how strong your hands are and the humidity where the instrument lives. Frequently played instruments need new strings sooner; instruments left idle still age through oxidation and changes in tension.
After restringing, pitch takes a while to settle and tuning will be frequent for a period. That is normal. Bring strings up to pitch gradually rather than pulling them straight to the note, which reduces the risk of breakage.
Wiping the strings before playing and keeping your hands clean and dry will extend their life to some extent.