This depends significantly by program but at AppAcademy the attrition rate was around ~6%.
I'm pretty happy with a/A though. I can't speak for every program, but it seems we cover a lot more than most. That said, I think people need to come into it with more to be successful - I know other programs take folks with absolutely zero experience.
Only a few of us had any kind of CS background though. I was one of another 16% in my cohort who had any idea about different data structures, Big O, etc.