My dd's dance school does shows every 18 months. They tend to work on exams and stuff between the shows. They also do a big competition once a year, but they have separate classes to prepare for that.
Luckily for dd, she goes to a drama group as well, and they do lots of performing which includes some dance/musical theatre etc. She also gets to do some dance in her big school productions - now she is at secondary, the dance numbers tend to be given to the kids who have had some dance training, rather than the primary school style dancing that's dumbed down so that everyone can do it.
DD is 13 now, and some of her dance friends are a bit older than that. They're all obsessed with dancing. None of them seem to have lost interest as a result of the shows being spaced far apart - in some ways, I think it just makes them more exciting. As a parent, I have to say I'm grateful for the gaps, as I spend a fortune on costumes/tickets/DVDs as it is!
I was an instrument player myself, but having watched dd do it over the last ten years or so, I must say, I think dance is a fab hobby. Builds confidence, teaches discipline, promotes good teamwork, provides opportunities for regular exercise etc. Not to mention the social angle. DD goes into her dance class all stressed and wound up after a busy day at school, and comes out like a different person.
She doesn't only do ballet, though - she does tap, modern, acro and some other stuff too. Nice to have a bit of variety in my view.