There's lots of factors to consider when trying to decide how many days is enough. We've visited DL with many different scenarios: seeing the parks as a couple, or going with toddlers or an infant, or someone in your party needing a wheelchair, or hanging out with teenagers, etc. Also, a lot depends on how crowded the parks are, what the weather is like, and how long the parks are open.
Our DL/CA plans are usually three days, sometimes four. We always ask whoever is going with us, friends or family, what is the top ten attractions or shows that they would like to visit during their trip. And we see to it that everybody gets to see their ten choices during the length of our stay. Any show or ride we see after that, we call a bonus! We always leave the parks each night with bonuses!
This trip we had a party of 5 adults and 1 infant with a stroller. So things were geared around our infant's comfort, which meant going back to the hotel in the heat of the day for a break and nap, and returning in the evenings for the parades, fireworks, and Fantasmic. We used the baby pass lots in the mornings.
The three days, two nights were just right for us this trip. Everyone saw their top ten picks at least once and in many cases, twice or more. We saw all three parades, 2 shows: Aladdin and Snow White, the 3 newest or refurbishes: Pirates, Buzz, Space Mt., Fantasmic once and fireworks twice. Plus, we stood in lines for character pictures.
DL was open 8:00am-midnight. CA was open 10:00am-10:00pm. Our hopper passes had one early admission for Saturday, but we didn't use it.
The few things we didnt see or do this trip: (DL) Mainstreet transportation, monorail, rockets, autopeia, Alice, HISTK. (CA) Orange swing, MM. Nobody in our group was disappointed not seeing these this time, so this three day trip was perfect!