Kinda depends:
1st time: A little less than 40 hours
2nd time: About 42 hours
3rd time: About 70 hours including completing everything and beating seph.
All times include adding bathroom breaks, being yelled at to do homework, clean room, chores, and sleep. Yup I guess you could say KH2 ruined my responsibilities list. My parents sort of don't trust me anymore because of it....oh well.

AT LEAST I HAVE MY KH SERIES!