I have never made it for more than three or four hours at AK...although I do pop in at each trip. This is from the view of an adult with no kids. My reasons:
1) I AM a zoo person, but I have been to much better zoos around the world. This one rates a one-time look at each attraction, but doesn't warrant a return trip.
2) The major attractions can all be completed by two adults in less than three hours. Once you have seen Bug, done the trail, they do not warrant a return trip. Dinosaur! The safari! Primevil whirl, now Expedition Everest, and most adults are pretty much done.
3) By early afternoon - this park much moreso than any of the other parks - just becomes a wild stroller kingdom. Many of us without kids just can't tolerate that after a few hours. The kiddie melt-downs seem to occur much earlier at this park. By two o'clock I am itching to get back to adult attractions at Epcot...
4) Of all the Disney parks, this one feels like it is the most crowded (I know it is not), the dirtiest (I know it is not) and crowded with the most souvenir stands (which might be). After awhile, its like wandering around a big shopping mall with the air conditioning turned off.