Keep it under 50 characters. Its general best practice to keeps subject lines to fewer than 50 characters. Subject lines with less than 50 characters have higher open rates and click-through-rates than those with 50+. Go over 50 characters and you risk being cut o-.

What is an appropriate subject line?

The subject line should communicate exactly what the email is about so that the recipient can prioritize the emails importance without having to open it. For example, writing “Do you have a sec?” is vague, says Augustine, since the reader will have to open the email or reply to figure out what you want.