About 80 to 90 minutes. You can, however, get off at designated stops and tour on your own. Later you can reboard the bus to continue your tour.
You can stay on the bus and complete the entire tour in under 2 hours....it will take longer depending how often you get off and see the sites at each stop. The buses usually follow each other in 15 minute intervals
If you dont get off at any of the stops it takes around 70-90 minutes.
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