In 1973 the National Parks Service established an area called the Golden Gate National Recreational Area (GGNRA).
In addition to Alcatraz, GGNRA includes the Presidio, San Francisco Maritime National Historical Park, Muir Woods National Monument, and other sites.
Their Day Tour option is the most popular package, This option includes a 15-minute ferry ride to and from the Island, as well as a self-guided audio tour to provide historical background and interesting insights into the Island and its history.
A 2-3 hour tour of Alcatraz Island should be sufficient to see everything on the island, and still leave you time to tour other parts of San Francisco during your time there.