You need to ask yourself "why" you want to use a compressor, which is a hard concept to wrap your head (not so much your ears) around. Most of the eq I use in the box is a high-pass filter to clear out the bottom.
I usually compress going in a smidge. In the box, it depends. Compression compresses the difference between the loudest and softest levels of a signal. Compression doesn't "make the signal louder." It makes the signal more even, which allows you to raise the average signal level via make-up gain if needed. However, the sound may "feel' louder since the average (as opposed to spiking) level is greater, making it sound fuller, even without make-up gain.
Sometimes a spikey, jangling acoustic is what you want to serve the song and compression is likely to work against that sound. But if you are using acoustic guitar as a rhythm guitar, compressing can give you a steady, even sound that performs well.
As said above, there is no "standard" setting for this or that. Different people (and engineers and producers) hear things differently and what works for big time producer X might not float your boat and be totally out of sync with how you want a sound to work, and the only way to do that is put in the time. I know that doesn't help you at this point, but the only thing I can say is keep working at it and use extreme settings and solo to hear what a compressor is doing to the sound. Then you can decide on whether to use it or not. It is kinda unfair: the more you record the closer you should be able to get a sound going in, which in turn makes it easier to get the sound in the box.
One of the best and easiest comps to use IMHO is the SSL buss comp. Put your guitar(s) on a buss and strap it on - the buss should be driving a bit, not a guitar-combined -24 dB signal. It gets very easy, again in my opinion, to hear the comp working. Say you are feeding 2 guitars to the buss and when you play with the settings you can hear the louder of them "pop out" above the sea of sound when one of them spikes some - but the PC buss comp does it very smoothly, naturally. Now put in the 1176 emulation and you will not get a nice ramp up and down, but an incandescent bulb turning off and on - usually we don't hear that on commercial releases (except for drums/perc). You need to approach learning compression in an experimental manner to get better faster (and of course, having chunks of time to devote to it).
Rather long post and I hope it helps. Basically, presets are a good place to start, but they don't know the tempo of the song or the level you are hitting the comp with. And those are the major determining factors in the settings to get a desired sound.
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