Short subject lines work best.

Web specialist John Childes advises you to make the subject line interesting, but to keep it short. Otherwise, you run the risk of the subject getting cut off in the inbox (especially on mobile devices) or losing the attention of the recipient after about 20 or 30 characters. "There just isn't enough space or long enough attention span for long subject lines," he says.

Source: Write Now, 2/12/14