What kind of trip do you want? Open to close, all rides, all day? Or do you want to enjoy things at a slower pace? Do character meals, shows, dining packages, shopping, etc.? If you want to go hard, all day, 3 days should be enough, especially if you purchase MP and use it well. If you like a more relaxed pace, 5 days is great -- that allows for shows, TS meals, pool time, etc. And what time of year were you looking at? Certain times of year are more busy and crowded than other times, so more days is good for managing crowds without feeling overwhelmed. If you want a whole week, throw in a day for the beach and maybe a day for some shopping and SoCal sightseeing.