I had a friend in college with no prior musical training who decided his second year to become a classical guitarist. We kind of teased him at first because he clearly had no sudden music-related epiphanies nor any idea of the time involved to master an instrument much less learn to read music and understand theory.
Nonetheless, he spent the time you'd think would be required on a daily basis to gain such competency, plus starting the music track classes. At the end of a year if I didn't already know his initial skill level and his general obsessive dedication to all endeavors, I'd have assumed he was a savant.
Put another way-- most amateur musicians I know spend less than 30 minutes a week doing anything resembling practice. I often hear people drilling a mistake in that scant time. Given that, I'm amazed most people can play anything at all.