As a focus, it sounds like 15hr/week is the bottom end of "dedicated" whether sport, music, or other. Which from age 5 to 18 ends up near 10,000 hours of practice.

My rough estimate of 3 hours a day on weekends, 1.5 hrs a night, and 1hr a day in class has DS8 at 18.5 hours a week doing math. Since the home part is self-directed, it's interesting to see that range line up from his own instincts. This year with math at school closer towards his level, his home time has dropped a bit.