I used to work as flight instructor in the US. Most of the instructors stayed for about a year before they moved on. It was a stepping stone for almost all of them so the school had a constant change of staff.
I think it's a shame since I believe a lot of knowledge and experience is lost that way. Instructors quit just when they start to get good at their new trade. They are then replaced by a new inexperienced instructor and the cycle repeats
