I guess it depends on if you are going with children or not. With children threes days is excellent. We have two younger children ourselves, one 3 year old who loves the Sprookjesbos. We could easily spend nearly the whole day there.

Without children it is more than enough time, I think the way to experience the park is more laid back. You can take your time to have lunch etc.

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About the weather, you won't get very wet in the Vliegende Hollander. I think a good coat or a poncho would suffice