According to a recent detailed report by AWeber, an email marketing software, 82% of marketers and developers send emails with a subject line including 60 characters or less. 60 characters are equivalent to 30 words. Although this may seem like quite a lot, varied screen sizes do allow for a maximum of 30 words, or 60 characters, to appear within the fully visible subject line width.
Having said that, the average subject line length is 43.85 characters, which is equivalent to 22 words. Many email service providers do not show full subject lines longer than 50-60 characters, so using fewer characters than this is advisable.
Something marketers need to consider with subject line length is that on mobile devices or smaller tablets, longer subject lines may not appear and users could get frustrated due to their screen size not presenting the whole subject line, due to limited space.
So, split testing your subject line length to see what works best for your subscribers is a great tip for those looking to improve on their subject lines and open rates.
Source: Convince and Convert (https://www.convinceandconvert.com/digital-marketing/email-marketing-statistics/)