I think we all have kind of felt that coaching turnover has increased in college football over the past decade as more and more ADs get less and less patient (to say nothing about job-hopping coaches themselves). This piece from Coaches by the Numbers lays bare just how high the turnover has gotten. If the thing looks a little tl;dr to you, just skip to the final four paragraphs for the bottom line. The whole thing is worth reading though.