Hi Angie -
I think the days of "your subject line can't be longer the X characters" are over, but you still want to convey your message in a way that is easily digestable.
Check out the reports from Mailer Mailer (http://www.mailermailer.com/resources/metrics/index.rwp) they've got some interesting results in their report.
From their report: subject lines with only 4 to 15 characters achieved the highest open rate of 15.8% during 2012. Longer subject lines resulted in lower open rates, with a very slight correlation between length and open rate.
Hope that helps!
-Sierra