The string tension on your acoustic guitar depends on a few factors. String gauge, scale-length and tuning all play a part but if you assume somewhere around 200 pounds of pressure, you'll be in the ball-park.
If you sat there with 200lbs on your shoulders for years, odds are you might begin to buckle a bit. Your acoustic guitar holds up better than you would but that tension can take its toll.
If you have any steel-string acoustic guitar for long enough, chances are good it'll need a neck reset at some point in its life. That string tension alters the geometry of the instrument and the most obvious way is that the action creeps up to a point were it's uncomfortable or awkward to play. When a reset will be needed is anyone's guess. Different guitars are, well, different. Could be five years, could be fifty.
Most guitars have some additional height in the saddle to allow it be lowered, taking the action down with it. This buys some time but, eventually, the same thing can happen. It's not unusual, on older guitars, to see a saddle that's been lowered repeatedly and is little more than a sliver, barely above the bridge.
Might be a good time for a reset, then.
A reasonable rule of thumb is that, the plane of the frets should be at the same height as the top of the bridge (that's the wooden bit and not the white saddle). Putting a longish straight-edge on the frets can show you what the story is, as in the photo above. As you can see, it contacts a few millimetres below the bridge-top. Sighting down the frets from the headstock can give you a good idea visually if you don't have a long enough ruler.
What happens in a neck reset?
Basically, we're trying to re-adjust the geometry of the guitar and neck so that straight-edge in the photo gets raised enough to touch or clear the bridge. That means changing the angle at which the neck joins the body.
To do this, the neck has to be removed and some wood taken off part of the heel.
A neck reset on a bolt-on acoustic
Ahh, a bolt-on neck…
Dubious arguments about tone aside, if your acoustic guitar has a bolt-on neck, it does make a neck reset a little easier. The first step, you see, is getting that neck off and the easier that is for me, the cheaper it is for you. Bolt-on necks mean less hassle trying to get glue-joints to release.