The AAA Foundation wanted to see how well parents did when teaching kids with learner permits. They put cameras on the dash of cars, and followed 50 families. They were disappointed to find that supervised driving averaged only 2 hours a week, and only 20% of that at night, where kids need more experience.
The foundation recommends 100 hours of supervised time, and taking time to practice in bad weather or at night. More details from the study are on the AAA site.
This clip from NBC Today shows just how important spending time teaching your kids to drive can be: