It took me a week to finish KH on expert mode. Mind you expert was fairly difficult especially when a boss can kill you in 2 or 3 hits, 1 hit if you're unlucky. The monsters were nasty as well. I'd be tapping the heal button every two seconds. At some stages, I thought some bosses were just impossible.
Oh yeah, regarding to someone's message on the first page, if you wanna beat Sephiroth, the idea is not to lose sight of him. You should use manual target lock (if you don't already), it's a lot better.