How frequently do you change your strings? It can be highly individual – some players have to change strings for every concert. That can be every day. If you are recording something in a studio you can even change strings during one day. In my case the steel strings last about 100 hours, nylon strings a bit less. So after 2 weeks they are dead for sure.
The first sign that the strings should be changed is loss of the bright tone. The next call is if they cannot be tuned any more. Last call is a broken string. Of course, a string can brake without warning at any moment. On classical guitar I always change the basses more frequently than the trebles. My set is made up from 3 different brands.
Strings are an interesting topic as strings are directly responsible of letting you to produce a nice and even sound (or not). No matter how fast the strings wear out, the process is always so smooth that it is difficult to notice when do they exactly lose the sound. So I recorded a little test. Here are to recordings: before changing strings and after. Note: this example is not played on a classical guitar!
The guitar is an average solid top dreadnought and strings are 0.12-0.54 bronze. Old strings were used for a few months and didn’t show any sign of being out of tune yet. But listen to the sound! Old strings had completely lost their high tones and even the trebles sound much different. In the sound of the new strings there is a certain ‘ring’ that almost gives an impression of a bigger reverb. Actually the recording conditions were the same.