I haven’t read Drawing on the Right Side of the Brain, but I’ve seen some impressive before and after photos, like this and these. Someone here might know more about it.
I guess it depends on what “learning a skill” means. Learning a skill to a level where you’re happy with the result might not take three years.
The book Ultralearning by Scott Young is good for inspiration. Here’s a video of his one month experiment with drawing:
If you search YouTube for “1 year progress drawing” (or another skill) there will probably be some results. Here’s one for drawing:
