I've seen turnover that bad at past employers. In some cases it continued for years after I left. I think it depends on the company, not necessarily averages.
Ditto for a place where I used to work. Unfortunately for me and my colleagues, the reason for the high turnover was that they kept hiring good dev managers who all quickly identified the projects' major problems and set about trying to solve them. This had the unfortunate side effect of revealing the incompetence of the TD, so they had to go quickly.