Actually, the letter scene takes all night (in theory, at least).
Other highlights: the opening, with Mme Larina and Filipyevna remembering their husbands; Olga's aria; what Onegin says to Tatyana when he's received her letter; the dance at Mme Larina's, with M. Triquet's little song in French; the Onegin/Lensky duel scene; the polonaise; Prince Gremin's aria; what Tatyana says to Onegin when SHE receives HIS letter; the subsequent carpet-chewing.
One of the great operas, IMNSHO. And the moral? "It's never the right time".