My last trip was three days during the off-season, and with the park closing at 6:00pm, I made it a half-day... but I probably could've spent another four hours to appreciate everything.
Some of the park is "educational" and it may depend on your interest in animals. If you enjoy seeing the exhibits, you could certainly spend a full day on just that alone. The Kilamanjaro Safari alone is worth multiple rides on the same day.
Even if you're not into zoos, AK is one of the most beautifully themed parks available. You truly feel like you are through exotic lands around the world. Expedition Everest is among the most fun rides that Disney has ever created. The food options at AK are excellent.
I definitely think AK is worth seeing if you've never been. Of all the parks, I felt it was the most relaxed and leisurely. My favorite was just meandering the many pathways and talking to the guides who were very knowledgable about the exhibits. Ideally, your AK day will be flexible so you can linger if the place inspires you. If you feel bored, you can just hit up the big attractions and leave earlier.