I have proved by actual trial that a letter, that takes an hour to write, takes only about 3 minutes to read!
Magnitudes are algebraically represented by letter, men by men of letters, and so on.
If doubtful whether to end with "yours faithfully," or "yours truly," or "yours most truly," &c. (there are at least a dozen varieties, before you reach "yours affectionately"), refer to your correspondent's last letter, and make your winding-up at least as friendly as his: in fact, even if a shade more friendly, it will do no harm!