Minimum, one hour.
It's up to you
Last time my wife and I went we spent 4 hours there and could have taken longer.
If you came in the morning on a weekday and set 2-3 hours you should be good. If you are around on a Wednesday plan to go in the late afternoon and stay for the free concert where people picnic. You can bring in food and drinks or buy from vendors there.
Couple of hours
It's alot to see and explore
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