Dc is a serious musician but has become more serious over time. At ages 8-10, he practiced 1/2 hour most days. By 11, he practiced 45 min most days. Now, at 12-14, he practices minimum 1 hour, 6 days/week PER instrument. That's on top of rehearsals for various groups plus music theory plus performances, which average another 6-8 hours/week.

I'd say youth orchestra would be a good idea as early as they'll take your dc. It teaches ensemble skills including listening and keeping a tempo, and it's generally more fun and motivating than solo practice alone.