I recommend a minimum of one hour. That is time to wander the house, and walk the Grand Alee as far as the casino complex. If you want to add time, walk the Alee all the way to the cliffs by the beach, and/or wander through the recently restored Italian gardens.
Depending on how long you would like to inspect ornate molding and fire place mantels or if you want to walk all the way to the cliffs I'd plan on half hour to sprint through it all or 1.5hrs to meander, and I'm not sure about prices/access.
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