St. John’s Cathedral

John's is completely unique in Dutch architecture in terms of construction and has been under constant restoration since 1858. With its double flying buttresses with the 96 flying buttress figures and the reliefs above the windows, the exterior provides a striking picture of the decorative instinct of late Gothic.

I would guess you should allow 3 to 4 hours for the tour. The Cathedral has been labeled the largest Gothic Cathedral in the world - The Vatican is larger in size but 1. it is not a Cathedral - the Pope's Cathedral is the Archbasilica of St. John Lateran in Rome, and 2. is not Gothic.