Too long. 20 years.
Edited to add: I had the "feeling" it was all BS at the end of the 70's, and talked myself into staying because of the "brotherhood", (yeah...right).
After the 1995 generation change, the feelings were confirmed it was BS, they were making it all up. 6 years later, during a meeting I just felt I didn't belong there, left and never went back. During those in between years I started paying closer attention to the Watchtower, and looking up scriptures quoted in context.
After leaving I read CoC, Penton's book and Carl's book, which basically confirmed everything I felt I already knew.