How hard is it to learn the acoustic guitar and how much time will it take learning it on your own (I mean to be able to play full length songs including riffs, strumming, etc.)?
It depends! :-)
Start with hand-friendly chords, a simple 3-chord song and a patient friend who already knows how to do this part pretty well. Mix in a little determination, a standard portion of hand-eye coordination and a reasonably good “ear” (meaning you can hear when the notes sound correct. Bake for 2 to 3 hours. Et voila!
One serious thing though- the process is more important than the result. Although this is hard to imagine when you’re starting out, it’s true. Enjoy your adventure!!!