It would probably depend on the type of walk you're doing. There are miles of trails/paths. You can power walk or wander thru gardens.
The garden covers 80 acres. They have a guided tram ride between April and October on Friday, Saturday & Sunday each week. The whole trip takes about 30 minutes. You can get off/on at any stop. This is all paved. The balance of the paths are gravel/mulch. It takes about 10 minutes to walk from the resort at the top of the hill to the visitor center at the base.
If you want to see everything and take some moments to observe and really take in the different gardens I'd say 2-3hrs.
It took about an hour to walk around without stopping
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