You've prepared yourself with method acting, reading the course material and with location scouting. Now do yourselves a favor and keep the sessions short and sweet. Really short, or they'll definitely turn into something less than sweet. In fact, 15 or 20 minutes each time out is plenty for getting the basics down without burning out or screaming at each other. The more often you can do these short sessions, the more likely it is the new driver will actually retain some of the wisdom you're handing down.
Another benefit to keeping it short? Your equilibrium. New drivers tend to make the car lurch about, which can make you (the passenger) lose your lunch. Take a motion sickness pill, strap on a pressure-point wristband, or bring along some candied ginger, because nothing says "You're doing a great job, son," like throwing open your car door and hurling on the pavement.