It's about a 17 minute ferry ride from Battery Park, then we spent 2 hours at the Statue of Liberty. It will take longer if you want to walk up to the crown, which requires a special ticket, or if you want to spend more time in the museum. I'd recommend taking the ferry to Ellis Island afterward as this is included in your regular ticket.
It depends of the season that you're going... 3 to 4 hours.
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