I think there are a lot of variables. It can depend on time of year (schedule of park), crowd level, how far away you are staying, how large your group is etc.
When we have young children or babies with us, I like a good 4-5 days in the park to slow down our pace, and allow for naps. Same with going with a very large group, or at a busy time of year. If the group is smaller or it is a less crowded time of year, then 2-3 has been pretty adequate.