We did 5 days. 2 in DL, 2 in CA, then spent the last day alternating between the two, redoing all the favourite rides and shows. And spent one morning at downtown Disney. (which you can get into without a pass) Next time we go, we will do 5 again, and spend more time at restaurants, character meals etc.. that we didn't do last time. We are not big food people, and just ate wherever was closest. But after seeing the pics people posted on the Disney foods, I want to try some of those!
We had 5 adults, 2 kids with us. I guess it would depend on how many people are in your party, to decide how many days would be best, and how busy the parks are at that time of year!
Have fun!