I know of one trainer that has been through 6 beginners and is now over 25 years old... never crashed.

That's a bit exceptional... Average is closer to 6 months for a trainer. 1 to 2 years for the 2nd plane when you have a GOOD student.

As little as 0.5 min per plane with a poor student.[&:] who won't listen[:@] and insists on not using the buddy box