I have a Martin acoustic guitar and it's usually kept in its case. I use it about three times a week. The area I live in can get pretty humid, but winter gets a bit dry. It's kept in my apartment - which has a heater/air conditioner.
So I was wondering if I need to use my humidifier all the time?
The best thing is to get a humidity gauge (either for the room or the case) and monitor the humidity (I think the usual range to aim for is between 40 - 60%), then to use the humidifier when the gauge is below that range (but some humidifiers also lower the humidity when it gets too high).
It's important to be consistent. It is often harder on an instrument to go through cycles of lower and higher humidity (such as it would experience if you humidify sometimes but not all the time when it is needed) than to maintain a specific humidity, due to the fact that the wood is shrinking and swelling over and over again
I totally agree with the above answer, in that the only way to be sure as to what you need is to get a hygrometer and check the humidity. If the humidity is too low, which happens reasonably frequently in heated houses during the winter, the best thing to do is get a case humidifier like this one and leave that in your guitar case. That will help maintain a normal, healthy humidity level.
If your humidity level is too high, you'll find that a small bag of silica gel will quite easily do the trick.
I live near Phoenix, which means our humidity levels are very low most of the year. I have two acoustics plus a semi-hollow body I have to keep humidified.
I use hygrometers in both acoustic cases, just to give me a visual reading of the conditions in the case. Plus, in both cases I have three Humidipaks, using their little bags to hang two in the sound holes, and one below the head of the guitars. They keep the air in the case between 45-50% humidity constantly.
I periodically need to recharge them, which is easily done using techniques found elsewhere on the internet. Search for "recharge humidipak".
Humidipaks also work really well when the air is too humid. Warm them gently to get them to release their moisture, then cool them back to room temperature and put them into the case. They'll absorb moisture until they're full, at which point you warm them to dry them out and restart the cycle.
Initially you'll need to go through the cycles more often as the guitar and its case get humidified or dehumidified. Once those have stabilized, the cycle will stretch out. The bags in my acoustics have been fine for about five months right now, even with weekly practices and playing the guitars several times a week. The trick is to close the lid as soon as you take the guitar out.
If you're in an environment that alternates between dry and humid, then you can leave them alone and they'll maintain themselves. They're a great tool for keeping the wood at the right moisture level, and are recommended by Taylor guitars for just this purpose.
