Each individual and family is different. When someone says "XXX is a half-day park," they have usually been to that park before, and aren't interested in repeating some of the rides and attractions. [That's the case for me at DHS - I go at rope drop and am usually done by around 1:00.] For those who are going to a park for the first time, though, I usually recommend a full day at each park.
I have been to AK many times, and spend two full days there during a one-week trip. But I have a very high interest in animals (have worked as a volunteer at my local zoo for 16 years) so everything is fascinating to me.
As others have said, for your first trip to AK, I'd schedule a full day. There is lots to do. Besides the "rides" like the safari and EE, there are three great shows (be sure to see the Lion King show and Flights of Wonder, an excellent bird show). Then there are the animal exhibits - two great animal "trails" to wander through and watch the animals. There are often zookeepers posted at the exhibits to provide you with info about the animals and answer questions.
Most people would advise you to skip Rafiki's Planet Watch and Conservation Station (you have to take a five-minute train ride to get there), but I really enjoy them. They often have live veterinary procedures you can observe through a window, and they have "touchable" animals like the petting farm animals and often something like a snake or other reptile to touch. There's also a small stage next to the petting farm where they do informal "animal encounter" type shows. Those are great - the zookeepers bring out animals and talk about them. I've seen Eurasian Eagle Owls, a porcupine, a hedgehog, some parrots - and they'll stick around afterward and answer questions about them.
So, see, I'm an "animal geek" and can find lots to do at AK. It's tied for my favorite park (tied with MK). For those who are only interested in rides, and/or are not very interested in animals, less than one full day would probably be enough.