I took 49 hours on the first quest. Then came Hero Mode, which took me about 29 hours. I just looked at my file, and it said 78:50. Skyward Sword takes a good amount of time, but it's well worth doing.
I finished my first playthrough in 38 hours 18 minutes, but didn't really do much at all. I haven't started Hero Mode yet, but I plan on doing it soon.
I have almost gotten everything in the game, but i decided to complete it sooner. If i would have gotten the rest of the heart peices and item upgrades i would have 70 hours. I finished with 55 hours.
edit: im going to start hero's mode tonight since i just beat the regular quest 8:01 this morning. So after im done that i'll put in my Hero's mode time.
I'm going for 100% with walkthrough, and since I watch the video walkthrough while I am playing it, (of course, paused), I have 10+ hours in the middle of the second dungeon.