I should think you'd have

-When you going back to kindergarten, Otis?" some one had asked.

-"Me? Day Bernice gets her hair bobbed."

-"Then your education's over," said Marjorie quickly. "That's only a bluff of hers. I should think you'd have realized."

Source: Bernice Bobs Her Hair by F. Scott Fitzgerald

I don't understand why the girl used "should think you'd have done" in the example. I understand that 'You would have realized' belongs to the third conditional. But why was it used with "should think".

I should think you'd have realized is not a conditional sentence. Should has a tentative meaning here, that is, the speaker uses it to express a degree of uncertainty that would have been absent in I think you realized.