Maisy,
It's interesting comparing your experience with ours - DD is also 'a dancer' (now 12), but for her the hours grew slowly but inexorably, rather than there being a single big 'jump'.
She started at 5ish, doing 1 hour. By 6, she was doing 4-5. 7.5 hours or so became the norm round about the age of 8. It varies each term, due to specific exams or preparation for different things, but 9.5-10.5 has been her 'normal' level for the last couple of years.
DD is not planning to be a dancer long term. This is the normal amount of dancing for all young dancers of her level in her dance studio. Some people do leave each year for dance schools, but many do not - it is a hobby for them, as it is for DD. The point is that commitment to any 'serious' hobby DOES become much more time consuming as children get older and more advanced - DS, just as a comparison, does 6 hours a week of music, not counting his own practising, and about 4-6 hours of sport, and he's very much 'recreational' level in both.
Both manage completely 'normal' lives as well, as do all the many young people who swim, play netball, play football, do gymnastics etc to a 'more than basic' level. DD plays an instrument, is in her school netball and hockey teams, is very efficient about her homework etc. DS can loaf for England.
I suppose what I am saying is that, while for you it is a big jump, there will be many many children who do this amount of dancing 'normally' without any thought of future vocational training, while keeping up normal lives as well.