It all depends on your own family. We spent 2 days at AK this year and had a great time (this was trip #8). DS loves animals though and could look down all those trails for half a day or more.
This year we slowed down and really enjoyed the parks. First time we had enough time to let them enjoy playing in the Boneyard - mostly in the fossil dig site. Got some great pictures of DS (who is 13) playing with his little sister (who is 8) that I know I will cherish as I know those days are soon drawing to an end. {sniff, sniff}
We spent lots of time both days wandering around the trails on Discovery Island.
We did the safari both days, specifically planning to do different times (one mid morning and one toward the very end of the day) to see what different animals were moving around at different times and we were pleasantly surprised at how many we encountered on the road and we even saw (gasp) the lion up and moving around - in all our trips we had only ever seen him asleep on the rock.
We did Nemo one day and Lion King the other without feeling rushed to run from one location to another.
We went out to Rafiki's and spent a fair amount of time (not always able to fit that in during our one day - get it all done trips).
We found some wonderful ladies (CMs) at a gazebo in Asia that were sitting and invited the kids in, the kids got to choose a charcter and then write their names on the paper, then each CM wrote the childs name in their native language and gave them some 3 x 5 cards of Asian countries.
We really enjoyed the relaxed pace. Got to see some/do some of our favorite attractions (Dinosaur, EE, Bugs Life) twice. Just enjoyed being together. Didn't feel like grabbing a spot for the parade and hour in advance was killing our schedule as we really didn't have one.
This trip, of course, was DHs R&R and he had to return overseas shortly after the trip and we purposefully planned 2 days in each park to slow down and enjoy it and not feel like we were running full out to get everything done that we wanted to. Looked around in lots of the shops, played with the instruments, etc in Africa.
Animal Kingdom is a beautiful park.