Honestly, I think at the younger ages, the least amount of hours you can get away with, the better. With overuse injuries being so common, I figure the less hours you add up over the years, probably better for the body. I think 3 hour practice for a 6 year old is the maximum. My daughter at 6 did 4 hr practices for a few months of level 4, but then they decreased it to 3, shortly after that she changed gyms, and she has always done less than her old teammates. She just started having 4 hour practices at age 10.
The way I see it she has many hours less wear and tear on her body since she has done less hours than she would have. Not that all kids get overuse injuries, but just my personal opinion on training hours.