Kingdom Hearts: Never, I watched my sister beat it and I got into the series that way, although I've defeated hard boss battles and solved difficult things that my sister could not, like beating Riku II and defeating Sephiroth, (ect). (Currently working on first time beating the game by myself.)
Kingdom Hearts: Chain of Memories: 1 time (I felt that beating it once was a good enough experince.)
Kingdom Hearts 2: 2 times (Once on standard and again on Proud I don't believe I need to beat it again, although I might beat it one more time, just as a refresher.)