You don't want to over-practice and wear the kids out but you certainly don't want to under practice and embarrass them either. For my two shorter African plays (each about 8 minutes), we practiced three times a week, one hour sessions, for two weeks. It was about a week too short. For the longer African play (15 minutes), we practiced three times a week, mostly one hour sessions but with two longer Saturday sessions, for four weeks.

The opera, with the addition of learning musical numbers, multiplies the hours of practice needed. The experts from the New York Metropolitan Opera suggested one hour of practice for every minute of the production. That was a lot of practice for a 40 minute play.