My DD is in 2nd grade and is doing 12 hours a week for level 5. She did compete old level 4 as a kindergartner and went 9 hours a week at that time. Her coach recently explained to me that while 12 hours is low, the risk of stress fractures, burnout etc is so high that taking it slow is the way she is training her. I was very impressed by this.
It has never seemed too much for DD (or DS who also does gym at a similar amount) but I can tell you that they grow up really quick in the gym. They become these mature little things and are very passionate and love what they do. The drawback is that they can have trouble fitting in socially in normal life. DD is incredibly outgoing in the gym and talks and lights up and her best friends are there. She is a completely different person at school--she doesn't seem to fit in this year (different from previous years). I think all the other kids attend fun fairs, play tball or bball for school, etc. and DD has no interest in those things. I try to see it as a positive but I'm just throwing this out as something to think about if you let her do those kind of hours early--it really does change them.