On the acoustic guitar as often as possible for that ideal acoustic sound. For most people who don't play very often I would say every 6-8 weeks will be fine. My general rule of thumb is that "if the strings aren't gold then change them". I get students all the time coming in with black strings on their guitars with dirty fretboards. They wonder why their guitar feels uncomfortable and chords are hard to play. I usually give them my guitar to play on so they can feel the difference. They get shocked and immediately want to get their strings changed. In my guitar lessons I will dedicate a full lesson to showing you how to change your strings properly and take care of your fretboard. ​

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On the electric guitar you don't have to change your strings as often but when you are playing lead guitar, you tend to break the higher strings when practicing guitar solos. I would say every 3 months is fine.