From An Email Minute
Different email clients allocate varying numbers of spaces to the subject line. For example, Hotmail limits the number of characters it displays to 45; AOL shows up to 51 spaces; and Outlook allows the user to establish how much of a subject line to display.
Research has consistently shown that subject lines less than 50 characters long generate higher open rates. That's at least partly because so many email recipients see truncated subjects when the lines run more than 50 characters.