CBT is hard to pass, hard to fail. Easy to need a second day.
From what I can tell, a lot depends on the school, and their approach. Some places, you get to lunch time and all they've done is talked at you about safety gear. You don't sit on the bike until the afternoon, and no surprise that you're unlikely to be able to get out on the road before its too late. Other places take a look and take the people who look half competent out and make sure they at least their couple of hours on the road.
Best approach would be to ask the school how many people get through the CBT on their first day and you'll get a good idea of how they run the thing.