Answer: at the same time as your E, A, D, B strings.
But really when should you change your strings?
If you’re somewhat new to playing a stringed instrument, this can be a tough one to figure out. There is no absolute answer to this question, as there are different preferences in tone, and different contributing factors to to how long strings last. In general though, it seems like most people should be changing their strings more often than they are.
Why do strings need changed other than when they break? Your standard guitar, mandolin, banjo, etc., strings are made of a steel core wire, with a few of the strings wrapped with a bronze wire. Check out this cool video from GHS strings to see the process: www.ghsstrings.com/videos/view/43823145
All that metal starts to rust and tarnish (yes, you have a rusty g-string that could break at any moment) whether you’re playing your instrument or just letting it sit in its case or on a stand (I don't recommend leaving it in the case or on a stand.) Humidity, sweat, oils and dirt from your fingers all accelerate that process. Dirt and oil and even some skin (gross) from those sweet slides you’ve been doing start to fill in the space between the bronze wrapping on your lower strings. These all contribute to the lifeless, dull sound that slowly takes over your instrument. You get to the point where you have to play harder to get the same volume, and still don’t have the tone. You’ll notice your fingers getting sore sooner partly from pushing harder on metal wires that have been stretched to their limits and are refusing to flex anymore (also making it difficult to stay in tune), and partly from the microscopic rust and string wear that tears at your calluses (breaking loose skin cells mentioned above.) This all happens gradually while you’re focused on playing the right notes, so most people don’t even notice it (note: focusing on playing the right notes while not noticing things is also bad for relationships. *future blog*)
A new set of strings will have a much brighter tone, more flex, and be easier to keep in tune. But like I said about preference in tone, some people love the sound of new fresh strings. They ring clear in tone, with a quick attack. Other hate new strings, and can’t wait until a new set is broken in and sounding dull. We have a live recording of Tony Rice playing with J.D. Crowe back in the 70’s. He breaks a string and changes it on stage, then asks if anyone has some lipstick he can wipe on the new string to dull it. That’s the tone Tony prefers, which is why his signature Martin Strings set is a Monel wound string. Monel winding is much less bright than Bronze.
One way you can tell if you could use a string change is to just look over your strings, especially the bronze ones. Do you see a big variance in the color along the string, past the nut, or bridge? In general if you’re playing every day, than you should be changing strings once a month. When we’re on tour, we change strings every show. It’s party due to the elements, humid outdoor events with more sweating kill strings fast, but it also helps prevent us from breaking a string on stage.
If you don’t practice as much as you would like, maybe a couple days a week, then you could extend that to 2 months. Much past that and your strings are probably starting to make your already difficult job of playing an instrument, even harder.
While you’re changing those strings, try something new. There are many different brands and types of strings that can help your instrument sound better. I play GHS Silk and Bronze strings, which have a thin string of silk wrapped just under the bronze, (oooh, a silk g-string) dulling out a bit of the brightness, and giving my mandolin a woody tone. If you’ve already got a deep sounding instrument, bright bronze might help liven it up. There are also coated strings, which can last 3 times longer that non-coated. The coating prevents the rust, and dirt from getting to the metal, but having a coating on the string starts it out a little duller than non-coated. Each person and each instrument will be different.
Next, when is it time to buy new socks?
The Acoustic Shoppe