Minimum one hour, but you could spend over two hours if weather is good and you'd like to explore and just "be".
About a good two to three hours depending upon the event
In addition to the created earthworks there is also a short video and a museum of tools and equipment, all interesting. On a nice day you can stroll around, sit by one of the little pools, and generally relax surrounded by the calming nature of so much stone.
An hour to walk the grounds and see everything but I could spend days there appreciating the art.
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