According to data from Return Path, 65 characters seems to be a sweet spot for email subject lines, which is about 15 characters more than the average subject line. When subject lines are 61-70 characters long, they tend to get read. However, most email subject lines are between 41 and 50 characters. Longer than 71 characters are typically truncated.
Best Practice: Test your email on multiple devices and see how the subject displays
Restriction: You may not have numerical characters (i.e. 1, 7, 10) in your subject line. You may write out the number (i.e. "ten")