The ideal length of time to plan to spend at the Staten Island Ferry in New York City depends on what you're looking to do and see. If you simply want to enjoy the scenic ride and take in the breathtaking views of the Statue of Liberty, Ellis Island, and the Manhattan skyline, a round trip on the ferry takes about an hour.
However, if you want to explore Staten Island itself, I recommend budgeting a few hours. Once you arrive at the St. George Terminal in Staten Island, you can take a leisurely stroll along the waterfront promenade, visit the Snug Harbor Cultural Center and Botanical Garden, or explore the historic neighborhoods like St. George and Stapleton.
If you're a baseball fan, you might also want to catch a game at the Richmond County Bank Ballpark, home to the Staten Island Yankees. Keep in mind that the ferry runs every 30 minutes during the day, so be sure to plan your return trip accordingly.
Overall, I would suggest setting aside at least half a day to fully enjoy the Staten Island Ferry experience, including the ride and some time to explore the island.