[GlennLewis]

since about half of my repair work in the winter consists of acoustics with just this problem. The other half is the body has shrunk due to lack of humidity.

The answer to how long it will take is, "it depends". Since your guitar is made of different of types of wood, and finish types vary as well. So, if it took a month or two or three for the wood to lose the moisture.... you won't really know how long it takes to rehumidify until your guitar is back where you want it.

In the winter time, just leave the humidifier running all the time to deal with fret sprouting. Don't worry about knocking the frets back in.

For a quick rehumidifying experience for a shrunken body, loosen the strings & lay the guitar out flat on it's back. Take a plastic bag and spread it out inside the guitar under the sound hole. Take a washcloth and get it wet (not sopping wet). Microwave the washcloth for about 25 seconds. Put that in the soundhole of the guitar laying on top the plastic bag. Put another plastic bag over the shoundhole, beneath the strings. Come back in a couple hours and re-microwave the washcloth. Do this a couple times a day. Usually in a couple days the body has resumed the original dimensions and you are back where you wanna be. In extreme cases like this one Takemine I was working on last month, it may take more than a week for it to get back to playable condition.

Don't over do it, take it slow. It didn't get that dry overnight, and it won't rehumidify overnight either.