If you have a rusted string, change all of your strings. If you have a broken string, I would suggest, at this level, change all strings only if the strings are more than one month old.
Strings get stale after a period of being exposed to the air. Thus, the sound changes after a while. At your level, you may not notice. The longer you play, the more noticeable it may become. After you can hear the difference between fresh and stale strings, I would suggest the following: if you break a string, change all your strings if the non-broken strings are more than one week old.
Of course, these are just general guidelines. The most important thing is to use your ears and your hands to determine what is best for your situation. Good luck and let us know how it goes!
Rich Moulton
