Do not let the humidity get too low. There is nothing worse for guitars than low humidity. Guitars should NEVER be subjected to humidity levels below 35% relative humidity without being aggressively humidified in the case. There is nothing that will cause more damage to a guitar than low humidity, except for maybe large amounts of blunt force trauma.
The ideal conditions for a guitar are 70 degrees Fahrenheit at 45-55% humidity. Any lower than that, humidity wise, and you should seriously thing of using a soundhole humidifier such as the Kyser Lifeguard. If you don't, you can look forward to your
acoustic guitar developing at best buzzes, or at worst cracks. Both acoustics and electrics will have their fingerboards shrink, which will cause the fret ends to stick out (wood shrinks, metal doesn't), which can be quite painful.
Unless you live in a swamp or start pouring buckets of water into your guitar, you will never over humidify your guitar. Low humidity is a serious problem (my shop does over 600 low humidity related repairs a year), so I would suggest that you at the very least keep your guitars in a high humidity location. If your strings are getting worn quickly, that is unlikely to have anything to do with humidity, but is more likely related to your body chemistry, and there is nothing you can do about that. Some people just have acidic sweat, and they kill guitar strings like you wouldn't believe.
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