KH games have never been all that long. I beat KH2 my first time through it in under 20 hours, same with KH1.
Yeah 30 hours does seem normal for KH. Although it took me around 30 hours in KH2 with all cutscenes unskipped. I was hoping KH3 would be longer w/ cutscenes considering its on next gen consoles (I thought Nomura said 40 hours atleast). Unless the leaker skips cutscenes and he said he explored for 4 hours that playtime disappoints me.