There isn’t one.
It depends on whether you want your users to read the page faster or you want them to like the page.
Research by Dyson (reference below) showed that users read web pages faster at an optimum length of 100 characters and longer. However, when asked, they prefer shorter line lengths and believe they read these faster (even though they don’t).
Dyson, M.C. (2004). “How Physical Text Layout Affects Reading from Screen.” Behavior & Information Technology, 23(6), pp. 377-393