A mandolin player once told me that she didn’t change strings until she broke one. I know other musicians who change their strings before every gig. The real answer is somewhere between.
Strings need to be changed because they get caked with dust and sweat. This makes them sound dull and sometimes hard to tune.
Below are some guidelines. You may want to change your strings more or less often. It depends on your budget and what kind of sound you prefer. I know someone with a vintage Gibson who likes that somewhat dead sound that older strings offer. On the other end of the scale, if you prefer a bright sound, you might want to change your strings more often than suggested.
At any rate, if you’re playing with the strings that came on your instrument, run to the music store and buy new ones. Your ears will thank you.
Weekly if you’re practicing a lot and playing one gig or more a week. When I was touring full-time I changed my strings this often.
Monthly if you play an hour or so every day or if your hands sweat a lot. (If you wipe off the strings after playing, it’ll extend their life.)
Every six months if you’re a casual player. Just pick up the guitar during the summer to play around the campfire? For that occasional church gig? Then every few months will probably do ya.
Once a year. Even if you don’t play very often, new strings will put life into your instrument.
When the strings discolor or sound flat. You don’t have to go by time. If your strings sound flat, it’s time to change them. It doesn’t matter if it’s been three weeks or three years. Likewise if they tarnish or look dark.