I have 2 problems with the idea of rating parks on the "X-day" scale:
1) Just because it takes a full day to do all of a park's attractions does not mean that the everyone can be entertained for a full day. Different attractions appeal to different people, and different guests choose to tour the parks in different ways. If a park has the ability to entertain guests for a maximum of one day, the majority of the guests are not getting a day's worth of fun. Even if it's an average of one day, 50% of the guests are still leaving the park somewhat unfulfilled. [Example: while doing everything at AK can surely take more than a day, a family with little kids will not be able to fill a day, assuming they must skip, say, FOP, EE, Dinosaur, PW, and it's tough to be a bug.]
2) Parks that have a lack of attractions will have longer lines, hence increasing the time spent in the park. Therefore, the amount of enjoyment one gets from a park is not necessarily a function of how much time they can spend there. Disney's parks should take at least a day for guests to experience, yes, but it's also important to find ways to maximize that time for guests. [Example: Someone who doesn't make efficient use of FP+, either because they booked late or aren't familiar with the best strategies, could spend a full day at DHS, largely because it takes so long to wait for each of the four rides.]
