We know that high or low levels of humidity affect acoustic guitar, but how long does it actually take before a guitar starts to get affected by humidity? Furthermore, what do you think are the extreme levels of humidity both at the high and low end?
If you have a guitar made from solid woods, and you keep it inside a hardshell case, and you have no humidity control systems inside the case, and you put the guitar in its case in an environment that is outside of about 40% - 60% relative humidity, you can notice a change in guitar geometry in as quickly as a week or two, depending on the action of the guitar. Serious changes in geometry can take a couple months (under the given conditions). Depending on the deviation from ideal RH, permanent damage could take months or years to occur. If instead the guitar is hung from a wall near a heating vent, then 2 - 4 months of a cold winter could cause permanent damage.
Woods used for acoustic guitar backs and sides are usually cured at about 50% relative humidity. That level of humidity is generally comfortable for both the wood and people (people might prefer humidity a bit lower, but 50% isn't too dry or wet for most people). That said, there are many places that guitarists live and work that are usually at a lower or higher humidity than that.
When the RH is higher than 50%, the tone woods of a guitar will absorb moisture from the air and they expand as they take on water. When the RH is lower than 50%, water evaporates from the wood and the wood shrinks. Either growing or shrinking can cause damage, but low humidity damage is more common because the RH can get quite low in a home with forced air heating during a cold winter. Very high RH isn't as common, but can occur in temperate zones.
The easiest way to check the wetness of a guitar is by knowing the action it is set to when it contains the correct moisture level. When the guitar is too wet, the action is higher, and when it's too dry, the action is lower. If your action is normally high, it might be hard to tell. If your action is on the low side, then when your guitar gets wet it will sound slightly less bright and it will be less comfortable to play. If your guitar dries out, it will feel faster but you'll hear more high end "zing" and you may even get fret buzzing. You can also sight down the fretboard and look at where the line of the frets intersects the bridge and saddle. On a dry guitar, the bridge will be below the fret line, and vice-versa on a wet guitar. If your neck relief is mysteriously greater than it should be, your fingerboard could be dry (as it shrinks, it adds relief), and vice-versa with a wet fingerboard (note that open rosewood fingerboards on electric guitars are affected by this!!)
Another way to keep track of the moisture level of your guitar is by weight. The weight of an acoustic can be noticeably changed by excess or insufficient water.
For at least 25 years, Taylor Guitars has had great information about humidity and guitars available on their web site:
I've owned a solid wood acoustic for a quarter-century now and absolutely the best and easiest way I've found to keep it in good playing condition is to keep it in its case and to use a two-way humidity control system. These systems are packets of a special gel that absorbs moisture when the air is too humid and release moisture when the air is too dry. They cost a bit more than sound hole humidifiers but you never have to check the humidity of your case or acoustic to see if you've overdone it or underdone it with the humidifiers. You just have to replace the packs every 3 - 6 months (depending on your local climate). If you've invested in a quality solid wood acoustic, the two way packs are a great bargain to maintain the health and playability of your guitar.
I put single gel packs in my electric guitar cases also to keep the rosewood fingerboards from getting too humid or too dry. I've seen rosewood fingerboards get so dry the wood changes color from a rich brown to almost oak-like. A single gel pack (acoustics use three gel packs at a time) and periodic oiling of your open rosewood electric fingerboards will keep your electrics in top shape.