I think that 3 days is good. Our primary objectives usually run as follows:
1) Get to rope-drop every morning, early entry if you have that option.
2) Use fast passes, use fast passes, use fast passes! (Oh, and use fast passes!)
3) Rides in the mornings, shows in the evenings. (We usually walk on potc from rope drop through 11am, even over Christmas, but afternoon it can be a half-hour+ wait easy... same with HM)
4) Avoid spending too much time walking across the park. Hit frontier land and do most everything you want to do there all at once, then move on to NOS, do everything, move. Without a plan like this you end up spending a ton of time walking and fighting through crowds. (Not to mention that it is like a 1/2 mile from the enterance of DL to Toon Town or across from Splash Mountain to Space Mountaion, and that all adds up quick!)
5) Use provided transportation. It is much quicker (and easier on the feet) to grab the train or trolly or whatever than it is to just hoof it.. and it gives you a chance to sit back and enjoy what's going on without having to try and dodge everyone in the process.