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.