I think the number of days, depends on when you are going and if you want to try to do everything (or nearly everything). It also depends on if you plan to eat at sit down places or just counter service.
Assuming you want to do most rides, I think three day would be enough, but they would need to be three relatively long (full) days. In my opinion, there is more to do at DLR than DCA - so probably plan for two days at DL and one day at DCA. However, the parks are literally across the plaza from each other, so Park Hopping is really easy and well worth it.
You'll need to make good use of FP to avoid waiting in lots of long lines (usually FP for Indiana Jones, Splash Mountain, or Soarin' are the best bets (i.e., best time savers).
If you are going when all three nighttime shows are running (Fantasmic, fireworks, and World of Color), then you need at least two nights to see the shows. You can probably do two in one night (fireworks and Fantasmic go are pretty doable as a pair) and then WoC on a second night.
If you like to take things a bit slower, want to explore DTD, and like to eat at table service places, I would allocate at least three days (maybe four). I've not done four days, but we go often enough that we only ride some of the rides (for example, we never do Splash Mountain).
