See, we have done anything from 4 nights upto 16 nights and I have never wanted to leave so for me the time there doesn't make it any easier.
I have another experience from one-nighters to 25 days stays. (luckily the one nighters didn't involve a transatlantic flight
The odd thing is that, trips are either too short, or too long.
The short trip thing was adressed, so let me talk about long trips.
We realised that after 20 days (or so) we are too tired to truly enjoy the trip. I don't know if we're homesick, or too tired, or if it is because we realise that it will be over soon (you don't get that kind of realisation when you're there for a short period of time. That moment when you look behind you and you know that there is more behind than there is ahead)
but there is a point where we can become cranky.
It's really odd, because we're in our favorite place on earth, but sometimes, it's just time to go home.
We also realised that the best recipe for a long trip is our ability to change activities.
We know that doing 14 days at disney then 7 days at universal would be a recipe for total disaster.
instead we do a full stay at disney's, then slice the stay up into 3 ou 4 days blocks. A bit of park, then a day at the beach, then some parks againn do a bit of universal (even if it means paying a throaway night at Uni), then shopping, and a maximum of 3 to 4 hours of parks on any given day.
Everyone is different, but this works for us.
A long trip may not suit everyone. Florida is tiring, and if it's a parks only trip, then it may be best to do a shorter trip and pack as much as possible in a 10 nights stay.
Long stays do not mean more parks, it means more rest time in between, and enjoying all the other things florida has to offer.
21 days of parks from sun-up to sun-down is just madness (IMHO)
So if I may amend my previous statement. If OP is looking for a mainly parks-centered trip, then 10 days each year might sound fun.
If they're looking for quality florida time and taking their time to enjoy at a slow pace, then 20 days every 2 years are the way to go
Not everyone will cope with a 3 weeks trip. And some will prefer a longer stay to be able to take it slow.
In the end it depends on each individual guests .