Wife and i spent an hour but i can easily see spending more depending on how much you want to explore considering that there are a couple hiking trails that branch out from the inside of the gardens. We didn't really explore those but imagine it could take some time if we had.
I would agree with the other guides on here. If you want to just go through and look at everything without taking your time, it could be done in about an hour. If you really want to enjoy the garden and learn a little, plan in staying at least two hours or so.
2 hrs to be relaxed and enjoy.
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