It depends on how busy it is, but for summer time it can take up to two days.
It depends on who's going. Two able bodied adults could knock them out in a full day easy. Aquarium on a lite day is like a 30-60min experience. The Legoland park is like 1/3rd the size of Disneyland maybe. Now if you are going with young children itight be a lot to do in one day. We had little kids and it was more than enough park for them over 2 days.
IMO you can do both in 1 day especially with most days aquarium closing later than LL
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