Medium, the collaborative publishing site, measured attention (how long readers stuck with an article) and found the ideal blog post is seven minutes long. Of course the goal isn’t to pad your blog posts with filler. The main takeaway is to write as much as you need – if your text is compelling, readers will stick with you. And if it’s not, length is irrelevant.
8 Rules of Success for Social, Email, and Blogs | Inc.com