I did all my instructing in the USA, I'm in the UK at the moment. I started as a low time 200 hour instructor and saw one or two come and go, from 160 hours. We trained a few people from scratch to CFI and they stayed for a while. Only a few stay much longer than 1000 hours, Usually for personal reasons, definitely not financial. I stayed because I couldn't get a work permit for anything else. I loved it even though I was working at least six days a week from as early as 6 in the morning to 11 at night for around $600 a week. I had it good and I had been doing it for 3 years at that time.