We had 1/2 day with the tour group we traveled with. It was enough for a good overview of this huge place, but not nearly enough to see most of it.
I spent entire day there and didn't see it all! I had to go back two more times before seeing all I wanted to see
3 hours aprox
The garden tour took us about 3 hours. It was well worth it and didn't seem long to us at all. Be sure to wear comfortable shoes. You will do a lot of walking.
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