I've practiced for a year 5 hours a day, but looking back, it wasn't very effective. I have come up with a two hours routine, and I've tried sticking to it for ten years. I'm a pro guitarist now, so I guess it has worked.
You said anyone with enough work could become a pro... I disagree. I'd say that anyone with practice could have a pro LEVEL, but being a professional musician involves a lot of things that don't have anything to do with music... I've jammed with guys that could shred me to pitiful chunks of meat with a guitar, but that were total idiots, and kept on turning their amps louder (that's an example) well, that kind of guy comes once on a session, but not another time. If you are a team players, have good social skills, and a small ego, you can become a pro. If you have talent and a lot of luck, you can maybe become a star. But lots of talented guys play in their bedrooms, because they do not have the authority and discipline it takes.