This is a typical schedule for Aviator. The first 12 aircraft are Duchesses, the rest are Cessnas and Pipers. As you can see, two airplanes are down for more than a week (red) and others for several days (yellow). The aircraft are constantly squawked for different issues, however the instructors DON'T squawk everything, because there would be no airplanes to fly!
On friday I back-sat a flight during which we had a door pop open at 5500'. This is apparently quite normal around here, and most instructors said that it's "good practice". Except we had to fly back to the airport to shut the door, and at the rate we pay, that poor guy paid around $90 for that wonderful "practice" time. During which, as you can see in the picture, he is hanging on to the door with a scared look on his face, and the instructor is flying the plane.
On a different note, an open door in the dutchess produces as much drag and yaw as a failed engine, so if you lose a left engine in flight AND the left door opens, you will have a bad day! (in this case there may not be enough rudder authority to counteract the yaw and roll of the aircraft)