Mornings are usually quiet. Right after lunch they pick up. Weekends they can get packed, but you can always start your visit at the exit and move backwards. That way you aren't stuck behind a large group.
I've found that it is different every time I go. One Friday morning, it was empty, the next Friday morning, they had a preschool field trip come and it was packed. Same for Monday mornings. Haven't tried weekends yet because I heard it was busy. Good luck.
We went early in the morning on the weekend and it was not busy. But in the afternoon it does pick up
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