Have been a few times in August but it's a bit too hot for Julie and Joshua so we stick with Easter from now on.
Can only comment on our own circumstances and on the holidays we have had but having been for 3 weeks in the past I don't think 24 nights is too long but I would say between 2/3 weeks (in our circumstances) is ideal length of time. Josh is ready for home by then.
Unfortunatley we are unable to go for longer than 14 nights now due to school holiday dates etc, but to be honest that kind of suits us, though a couple of nights on top of that would be nice.
24 nights I would definitely do a split stay, and as I don't feel I could stay in hotels for that long personally I would do a villa/on site type of holiday.
Just to give you variation in accomodation. A villa is ideal for relaxing and gives you privacy and all that extra room, makes a great contrast to an on-site hotel.
If it were us going for that length of time I would definitely look at getting away from Orlando for a while, I am sure we would plan a few days on the coast somewhere, I just don't think we could spend all that time in one place.
Staying on site for the whole holiday would be way too much Disney overload for me.
We also enjoy the other parks such as Sea World and Universal just as much as the Disney one's so you have plenty of time to do these too.
If it were us we would probably stay a few nights on site then have a few days at a coastal resort , maybe go back to Naples or Fort Lauderdale and then back to Orlando in a villa.
Like you say the Universal hotels are definitely worth considering, they look very special places to stay.
There is so much more to Florida than the parks and we have been fortunate to travel around most of the state over the years, it's great to go to other places too....though naturally that's your decision, not mine!