You should try to
- keep the Subject as short as possible, however.
Email clients can only display a limited number of characters for the subject in mailbox display. While this depends on the user's settings, you can safely assume that about 40 to 60 characters should be displayed.
Overly long Subject: lines not only irritate the email client, they probably also puzzle the recipient who expected a short summary, not the whole text of your message in the Subject.