For me it was somewhere between 5-7 years from when I started having serious doubts to when I finally left.
During that time my life got increasingly difficult trying to pretend to be a good little witness as I became more and more convinced of what a massive fraud that the WT was/is.
Even still, for the first 2 years after I left I was plagued by personal doubts and anxieties. It took a long time to really get strong again. In fact, it's still on-going.
Daniel