The duration of a "Hop-on Hop-off" bus ticket in San Francisco (such as with Big Bus or City Sightseeing) depends entirely on the specific ticket tier you purchase, but it most commonly operates on a 24-hour or 48-hour basis rather than a "calendar day." For example, if you buy a "Classic" 24-hour ticket and scan it for the first time at 2:00 PM on a Tuesday, it remains valid until 2:00 PM on Wednesday. This provides much more flexibility than a "one-day" ticket, which would expire at the end of the first night. However, you must be mindful of the operating hours; most buses only run from approximately 10:00 AM to 5:00 PM. So, while your ticket might be valid for 24 hours, you can only actually "hop on" during the daytime window. Some basic or "promotional" tickets found through third-party sites like Groupon might be restricted to a single "Calendar Day" (expiring at midnight), so it is crucial to read the fine print on your voucher. If you plan to see the city's major sights like the Golden Gate Bridge and Haight-Ashbury, the 24-hour model is generally the best way to ensure you aren't rushed.