Physically, at least 50% leave. Emotionally and/or intellectually, the number is 80% and up, in my experience.
I was always Stumbled™ by JW's that were my age that had been raised in it. It was obvious that most of them were there because of inertia and family ties, and nothing else. Some of them even seemed to say -- in so many words -- "you converted to this religion??"