I'd say 3 to 5. I did 2 separate tours.
You could do five hours if you have lunch, take your time, and do the boat ride.
We are fast walkers and spent 2 hours without seeing all the gardens. Early morning best to beat the heat
We spend about 5 hours
At least 3 to 4 hours
Spend all day.
You can spend all day, depending on what you want to do
3 to 4 hrs at least. Spend the time walking the nature trails
Minimum 3 hours. We also toured the house which would add another 45 minutes to an hour. If you add additional tours, you will certainly want to plan for more time.
I worked there, so I had all the time needed to enjoy everything there was to offer. If I was visiting I would plan for a minimum of a few hours to walk the grounds unguided. If you want to take the train, boat, and/or house tour, I would add an hour for each tour I wanted to do. If I was back in the area, I could easily spend an entire day there with no tours.
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