I'm pretty sure it's three hours starting from the first entrance time. The two times I saw it in Boston, I was not able to get the first entrance slot. You kind of have to let go the idea of "seeing everything" and just take in deeply what you do see. I know it's weird, but the fact is, like in a dream, there are new things to see all the time, and you never see everything because you wake up.
If you like it enough to return to the dream world, (ie buy another ticket and see it again) there will be things you see again, like a recurring dream, and dozens of things you missed the first time. It's thrilling and frustrating and completely encompassing. Like a dream!
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