It was about 40 min or so. Highly recommend it. It's great history!
Things to know before you go gor mint tour. It's free tour but you need to get a tickit. There are only few slots in the day. We visited it in the slot starting at 3.30 PM. For the tour starting at 3.30, you have to be there at 3.00 PM. You can't take purse, bags, back packs, food and drink, lighter, matches, weapons inside the place. And there is no locker room provided. So, you have to leave your bags in side the car or at your home. However, palm sized waller that fits in the pocket, powered off cameras/cellphones, umbrellas, walkers, wheelchairs, empty water bottles, and medical necessities are allowed.
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