I'd say when an exhibition takes on the dimensions of a rock concert or
sporting event, you've got a blockbuster on your hands (i.e., ticket
scalpers on the stoop outside the museum with signs reading "2 TIX- $150",
when the face value of the tickets is 8 bucks!)  Another dead giveaway of a
blockbuster is when every friend of a friend of a friend you've ever had
comes out of the woodwork to ask you to get them in to the exhibition since
tickets sold out months in advance.  :)

Nora Weiser