I am a regular at
Legoland and the Wild Animal Park. Open to close is the answer for Legoland. Since I am a member...I tend to go for 3-5 hours, but whenever we go with visitors, it is a whole day affair.
The Wild Animal Park really is amazing/one of a kind experience...but most kids won't make it a whole day. The displays are very spread out which allows the animals to live in wonderful spacious areas...but any kids we have taken is pretty bored after the tram/train ride around the enclosures. There is giraffe feeding which is pretty amazing in the heart of Africa exhibit.
Get to the WAP EARLY!!!! It gets pretty beastly hot pretty early as it is so far east.
So, short answer...LegoLand...all day. WAP first thing in the AM thru lunch and early afternoon.
Have fun!!