The DTD area is much smaller than the one in Florida. I'm not sure you'd need much time at all, maybe after park closing to look around.If you are going solo, I'd recommend staying on property, if you can afford to do so, or staying off site and renting a car to park in the Disney parking structure.
A hardcore, do everything Disney parks fan needs 4 - 5 days for a first time trip. I did 5 last year. A not so hardcore person, or those that don't do the more intense rides, would probably only need 2 - 3.
Someone else here could speak better about when to go, but I would imagine a non-holiday weekday when school is in session is probably a more optimum time to go if you want to avoid crowds. That or any day it is raining