Odds of Hanging On
One of the reasons the hiring process is so elaborate is so that the CIA can be sure that it's putting only the best individuals out there in the field. Therefore, if you're in, you're in...as long as you don't go get yourself arrested or killed.
Of course, you may also decide that you can't take it anymore, and leave of your own accord. The stress can be unreal, and some simply can't cope with the constant separation from family. They don't want to look back on their lives in thirty-five years and realize that they're all alone with nothing but secrets they can never tell, and no one around they could tell them to even if they were allowed.
Remember when you found out the twist at the end of Shutter Island and you couldn't tell anyone who hadn't seen the movie yet? It's just like that. (It's not at all like that, but you get the gist.)