On average, it's recommended that you set aside a minimum of 2.5 hours for your visit to Alcatraz Island. That time includes taking the round-trip ferry, which is about 15 minutes each way, touring the prison, and walking around the island to explore the gardens and displays. Of course, you're welcome to stay as long as you want. Just make sure to check the schedule for the last ferry back to San Francisco.
The ferry for Alcatraz Island leaves from Pier 33 at Fisherman's Wharf. While there's street parking with meters, finding a spot and having enough metered time could be a challenge. It's best to leave your car in one of the more than a dozen parking lots that feature over 3,000 spaces. The most convenient lot, which is at 80 Francisco Street, is only a block away from Pier 33.