Depends on how much time you have. We spent about 2 1/2-3 hours with small boys. Had to feed almost every animal and pet them all.
It honestly depends on how long you spend with each Animal. There's so many to see and pet. You can also eat lunch and shop as well. We spent between 2-3 hours and that was without children!
During my recent visit we planned 3 hours and stayed around 2. We didn't linger too long at any one place. As their web page says it's fair to bring a pack lunch.
Hour maybe a bit lonnger
An hour plus if you feed the animals and pan for gold. Worth the walk!
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