How long would it take to learn basic guitar (acoustic) techniques?
It depends… on how much you practice.
It depends… on what you want to learn/what your goal is.
Arguably, more important than practicing, your level of skill and/or satisfaction with the acoustic guitar (or any guitar) depends on ultimate desire. If you want to reach Rodrigo y Gabriela proficiency… it’s probably going to take you 10,000 hours (yup – that’s 20 hours a week – of practice – for 10 years).
As I said, it depends on your goal. If you want to be able to play your favorite songs, strum the chords, sing along, and impress your friends (and the ladies), it will take MUCH less time. You can accomplish this in less than a month.
If you are a quick learner, it can take up to several months of constant practicing to learn to play the guitar.