Probably it did not last for more than two or three seconds of real time.

The maid's second knock just prevented the revelation of the name of "Messrs.

---," who, like the lady in the mantilla, were probably non-existent people.

{13} Thus dream dramatises on the impulse of some faint, hardly perceived real sensation.

And thus either mere empty fancies (as in the case of the lost securities) or actual knowledge which we may have once possessed but have totally forgotten, or conclusions which have passed through our brains as unheeded guesses, may in a dream be, as it were, "revealed" through the lips of a character in the brain's theatre-- that character may, in fact, be alive, or dead, or merely fantastical.
A very good case is given with this explanation (lost knowledge revived in a dramatic dream about a dead man) by Sir Walter Scott in a note to The Antiquary.