Seriously? There are tots who practice 3-6 hrs a day on piano.
Yes, it was a serious question. 6 year olds practicing 6 hours a day is outside of my realm of experience. For some it's the price of greatness, for others it's child abuse.
Somewhere I read that Paco de Lucia practiced 12 hours a day in his childhood, and dropped out of school at 12 because it interfered with his guitar.