Its surprisingly quick.
You'll be at the final boss at level 30ish in this one.
Actually all of the KH games are relatively short, KH2 was longer because of double visits, but a lot of it was filler.
KH1 can be beaten in 15-20 hours, KH2 probably 20-30 hours if you're not speed running.
KH3 feels shorter because people are gunning for the ending and not taking the time to explore the environment and find Hidden Mickeys. Take pictures, go treasure hunting and do some side quests, and it feels pretty lengthy. It helps as well to play in increments to space out the play time, every world is about 2.5 hours, so if you really get immersed, it's double that.