How much time should I typically reserve for the visit?

Depends on how many parks. 1 FULL day for each park

3 to 4 days there is so much to do and see

2-3 days

One full day is fine if you want to enjoy for a complete day. Or even a weekend may be. But if you want to really explore every single thing over there, then some people stay 3 days to even a week (but for me that's too much)

4 days to a week.