Before you set out booking your hotel or buying your tickets, you probably want to know how long a vacation you're going to take. Everyone has different priorities, pacing, and time allowance.
I will be listing two numbers for each park:
- Minimum time you should spend.
- Ideal amount to experience the whole park.
Magic Kingdom: Minimum 1 day, ideally 2 days
Magic Kingdom is the park everyone pictures when you mention Disney World. Beyond being constantly filled with people, it's also the largest park at Disney World in terms of amount of attractions.
Trying to stuff everything at the Magic Kingdom into one day is incredibly difficult and requires heavy advance planning and a low crowd level. You can get to all the big things in one day (the mountains, Haunted Mansion, Pirates, etc) but you will probably have to skip out on smaller attractions or shows. Spreading the Magic Kingdom out over two days lets you not only enjoy all it has to offer, but you're not running in circles trying to check off everything. Two days also lets you make FastPass+ reservations at more rides and gives you more chance of getting one of those hard-to-get reservations like at Seven Dwarfs Mine Train. In extreme circumstances, like busy times around Christmas or if your family has to travel at a slower pace, 3 days may be necessary.
Epcot: Minimum 1 day, ideally 2 days
Families with small children will probably be able to get through Epcot in a single day, as many of the rides have height requirements. But for everyone else, I really do think you want to spend 2 days at Epcot.
You'll really want to visit Epcot more than one day if you're visiting during one of the many festivals the park hosts throughout the year. With tons of booths throughout the World Showcase serving all kinds of food and drinks, there's no way you can try everything on the menus in one day. I wouldn't say it's "trip ruined" if you can't visit multiple days, but I think Epcot is by far the best park you can 'relax' at.
Disney's Hollywood Studios: Minimum 1/2 day, ideally 1 day
Hollywood Studios is the world's most magical construction zone. The massive Star Wars land named "Galaxy's Edge" will not open until late 2019. However, some good news is that Toy Story Land is finally open. You'll probably be disappointed though, as there are only 2 new rides (in addition to the long standing Toy Story Mania whose entrance moves into the land) and a small quick service place (with all outdoor seating).
The park is still home to the largest collection of thrill rides in all of WDW. Tower of Terror, Rock 'n' Roller Coaster, and Star Tours are thrill junkie must-do's. There are also several stage productions, but beyond Indiana Jones they're all pretty mediocre. Unless you ride everything and see every single show, a full day probably isn't feasible. I'd pick up a park hopper and go over via boat to Epcot for the afternoon & dinner, then head back in time for either Fantasmic or the Star Wars fireworks, both of which are some of the best nighttime entertainment at WDW.
Disney's Animal Kingdom: Minimum & ideally 1 day
With Avatar finally opened, Animal Kingdom becomes a full day park. It is the most intricately themed of the four parks, but simply just doesn't have much to do. It feels like there's a lot to do, but only because everything is a mile apart.
Disney has made great efforts to make the park become a different beast at night. For instance, Kilimanjaro Safaris has a completely different storyline and they even mimic a sunset on the plains once the sun goes down. They've also introduced "Rivers of Light" a nighttime show which is the biggest debacle Disney has had in recent years. I personally wouldn't bend over backwards to see it, but it's still nifty if you can fit it in.
I think an extra half day could prove useful to take in all the little details of the park, but I don't think it's necessary to see everything attraction wise.