Generally, if they start to turn black and especially if they rust. :)[br] [br]But it really depends on your environment, how often you wipe them down after playing, and also if you wash your hands before playing. After many years of playing, I can hear a tone change after just one day, even after a couple of hours of recording, but I am very picky about having clean strings. I do everything I can to keep them as clean as possible.[br] [br]Some strings are made to last longer, like Elixirs, but they're more expensive. But they do last longer for sure. On my acoustic I have some coated strings that I honestly have not changed in years, but I rarely play it. If I were to start a recording project I'd proabably restring because I like a bright tone.[br] [br]If you're a beginner and playing regularly, like hours every day, you might wanna try changing every week or 2 weeks, I would think. As you advance in experience, you might be changing every week or every three days, because you'll probably develop your own feel for how dirty you can tolerate.[br] [br]I know some professional players who will NEVER change their strings as long as they don't break. For some, it's a tone preference, but I personaly can't stand dirty strings like that.[br] [br]As a pretty strict rule I have all the GuitarTricks instructors change their strings before we shoot. Otherwise, the strings will lose some of their reflectiveness on camera (and make them a little harder to see), and you'll start to see finger oil marks on them. That's always a sign that the strings have been played a lot.[br] [br]It really depends on so many factors! If they start to feel too dirty for you, try changing and see how it feels. Eventually, after changing your strings many times, you'll develop a sense for it. I can simply tap a string against the fretboard and tell immediately if it's too old for my picky tastes. :)[br] [br]-Carl.