Last month was our first trip to DL since 1997, so our timing was about the same as yours. I got 5 day parkhoppers, since it wasn't that much more than the 3 day tickets, and I wanted the flexibility to come and go as much as possible. That said, DS was "disneyed-out" at 3 days, 4 days was great for DD, and I still had a few things I wanted to do on day 5 (which I ended up doing alone

).
Things to consider: will you be getting up and to the parks early, when you can get more done in less time, or sleeping in and getting there with the crowds mid-morning? It also depends on if it's just you, or family, or a bigger group. If just you, you can pick and choose just what you want to do, and you can also make use of single-rider lines for the bigger rides. With family it might take longer, depending on their interests and activity levels -- e.g., kids stopping for character sightings will take longer, older folks who walk more slowly (um, I mean, savor each moment) will take longer, teenagers not interested in Fantasyland will save you a bunch of time. With a big group, almost always it will take longer, just to maneuver through the parks or get together at a planned time and place

)
I think 3 days is minimum to hit the highlights (2 days @ DL and 1 DCA), but I really enjoyed having 5 days to do things -- like the Billy Hill show at DL and the Animation building at DCA -- that I otherwise would have skipped due to timing constraints.