The answer, of course is: "That depends..."
If you know already that you are doing a 2-week summer camp and only see the children 3 hours a day Monday-Friday --- you do the math. 2 weeks x 3 hours of rehearsal per day = 15 rehearsal hours. That's not very long! Those few hours also have to include some breaks and "get-to-know-you" time. So obviously, you won't be choosing to do a long complicated musical!
With enough planning, big successes are assured!