so i know a kid with a trashy voice that i want to do dual vox with me for a pop punk project. do you think i could just hand him a guitar and he could pick it up within like a month? pop punk is pretty easy to write so i figure he can probably just teach himself.... worth it?
If you want it to sound good, than probably not. Seriously, I'd be seriously impressed if someone could become competent enough with rhythms and playing power chords really smoothly after a month of playing. Sure the music can get easy to play, but that doesn't mean that you can just hand anyone a guitar and they can do it.
It took me 6 months to get to an okay ability in terms of powerchords and such, but I had no instruction, and I'm semi-retarded in terms of learning abilities. I can see someone being good at powerchords in one to two months if they practice hard and focus on that as an objective. Rhythm is more complicated. I had it pretty much from the start, and I could easily do complex rhythms a couple of months in, but it could take several months to a year.
It depends on how much time and effort he puts into it and how much natural musical ability he has.

It will probably take him a few months to get decent at rhythm. But I doubt he'll be able to write any good songs at that point.

Does he sing already? You just said he has a "trashy voice". If he does why not just have him sing while you play guitar?
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