Depends on what one means by "large portion" and "success".
I spent a year at PIT, and I would easily spend 8 to 12 hours a day on drumming, between classes, lessons, workshops and practice.
Is a year a large portion? I don't know. Does that count as 8 to 12 hours? It depends on your perspective.
Success is largely a relative term. Are we talking income? Making a full time living? Or just artistic success? If you record an album that doesn't get released, you still had the skills to perform properly on the recording, but it's hard to say it's a "success".
And of course, different people learn at different rates. I once had a student who took weeks to learn just to learn one beat, while another kid the same age could learn anything I threw at him with in minutes.
As for dedicating 8-12 hours a day, it does happen. People like Weckl, Bozzio, and the such do practice that much. But in part, because they can.
It's the problem when at the time of being younger up and comer, is the guys that get a well paying gig/tour/band situations can afford to practice all day long in between tours/shows/albums, while the rest of us are busy working day jobs trying to make rent, which cuts into available practice time.