Whichever comes first, average strings should be changed every 100 hours of practice or every 3 months. But don’t worry, these strings won’t snap if you weren’t able to change them in this given time. In fact, the strings would last twice this long, sometimes even more. The rule of thumb is, you can still use them as long as it does not break. But of course, you would want a fresh and new set of strings every once in a while. If you are using your strings for several hours every day then you might want to change them before the given time period. But for those casual players then you need not worry about changing them every time. Besides, why care to change if you don’t really play that much. Also, you might want to change the strings if you notice corrosion build-up within it. It affects the sound of the guitar and can be due to a number of reasons. If you sweat a lot then corrosion build-up might occur more often.