Right off the top, an interesting finding: the analysis finds no correlation between #subject line length and read rate. That stands in apparent contrast to research from MailerMailer last year, which had linked shorter subject lines to higher open and click rates (though significance was not tested).
In the Return Path study, subject lines with 61-70 characters had the highest read rate, of 17%, almost twice the read rate of subject lines with more than 100 characters. Both were not heavily represented, though, comprising just 6% and 3% of messages analyzed, respectively. The most commonly used length was 41-50 characters, in one-quarter of emails analyzed.