I get asked this question a lot and I’m not going to be one of those annoying people who simply says: It can be as long as it needs to be.
I know you want straight answers so I’m going to give you something solid to aim for AND I’m going to base it off my own experience.
When writing marketing emails to build a connection with your subscribers, you don’t focus on selling immediately. You’ll need to use the art of storytelling to help your readers to know, like and trust you. This will build up over time, so you need to be consistent in sending out your emails to keep the momentum going.