I recommend at least 4 days for the parks. Given your username includes the word "seniors", I'm thinking you might prefer the better pace that would give you. Even for me (and I run 10K most days), it is very full days. Sometimes I just like to take a more relaxed pace. There is so much to do there.
So, 2 days is better than none, 3 is better than 2, and 4 is better than 3, etc. I have had as much as 8 day parkhoppers on one trip - and never felt bored. We did other stuff too (one day at Knotts, one day at Universal, one day at Sea World, one day at San Diego Zoo, etc), but having those hoppers where we could pop in for an evening after doing something else was great. I think we did about 5 or 6 full park days and just used the other days of the hoppers to pop in for a few hours.