Our friends over at Zelda Elements have reported on video interview with Zelda director Eiji Aonuma conducted by IGN. While there are quite a lot of things talked about in the 8+ minute video, one thing that stood out was that it was said that Phantom Hourglass was 80 hours long. He states that just because it is a handheld does not mean it is a short game.
Personally, throwing out these arbitrary numbers is just bad. We were all sorts of estimates for Twilight Princess and some players ended up beating it in 25 hours, while others have gone well over 100 and still didn’t beat it. At least saying 80 hours probably means it is a relatively deep Zelda game. You can watch the video at IGN or at Zelda Elements.