When you change strings is dependent upon how often you play, your playing style, your care of the strings, and the effect your hands have on the strings. The short answer to this question is between 15 to 30 hours of play! Here's why: As you play, the strings will collect dirt and oils from your hands. This causes the the strings to become dead and to vibrate unevenly along their length (also causing tuning problems). Dirt and oils get into the windings and down inside to the core of the strings where it can cause corrosion of the core. If your hands are dirty or if your hands perspire when you play, this will shorten the string life. Wiping the strings off each time you are finished playing will help prolong their life. Also if you bend strings or play hard this will cause flat spots on the strings especially at the fret locations, shortening their life.
source : musiccenters.com
