i think it would take maybe about 80 hours..............
100 or 120 if u try to do all the sidequest extras, beat side-bosses, get ultima weapons for everyone...........
i only say this because it took me 56 hours to beat kh1.......... actually i'm not sure if that was 56 hours to beat the storyline or 56 hours to go all the way up to level 100 everyone and 9999 stats for all characters........ funny story about that.......... (i wanted to watch asas on my tv since it looked better than the computer so i would always go and beat ansem and i didn't realize that i leveled up each time i did it........... i thought that i would level up but it didn't save and eventually i saw that sora, donald, and goofy were all at level 100 and had 9999 stats with all their great weapons[save the king, save the queen, ultima weapon])