We went at Easter this year.
We went for 12 nights.
We stayed in a villa in Kissimee, 15 min drive from Disneyworld. I wanted to be able to get away from Disney and I LOVE DISNEY but even I couldn't cope with 12 nights in a Disney Hotel.
We did 3 days at Magic Kingdom, 1 day at Animal Kingdom, 1 day at Hollywood Studios, 2 days at Universal. We did 2 days at parks then had a day off in between when we would go to a water park or something like that.
On park days we arrived when it opened and would stay till lunchtime/ 2ish then go back to the villa and chill by the pool.
Personally I think we could have saved Universal for another trip, but we went with family who wanted to do it. It was really good but my DDs were a bit young for the Harry Potter (8 and 5).
Do make a plan of what parks you want to visit on what days and book your fast passes up in advance (you can book them 30 days ahead, 60 if you are in a disney hotel). My advice on fast passes is to book them for say 11:30 onwards when the park starts to get pretty busy as you can get on a lot of rides first thing without queueing too much.
The food in the parks is expensive and dreadful. We took a lot of snacks with us for the girls so they could graze and then would buy some stuff at lunchtime to share. We would BBQ at villa in the evening which was v easy, gas BBQ, supermarket across the road selling everything you could ever want.
The Disney Water parks are amazing, I would definitely say get tickets with the water park options, we went there on days off the main theme parks and it was such fun but they weren't over crowded and the food there is actually really good.
You will have a fab time, my girls said it was the best holiday of their lives, I cried when we left!