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
Lisa Marie Ortega 
Interesting topic give the fact that web sites aren’t the same as printed media. And have even tweeted about certain articles that tout user experience best practices while doing so within an annoyingly wide 1000px column of head turning text.
Regarding print, here’s the Robert Bringhurst highly regarded take from his brilliant Elements of Typographic Style:
“Anything from 45 to 75 characters is widely regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal. For multiple column work, a better average is 40 to 50 characters.”
And to his credit, he acknowledges the web, liquid widths, and writes a slightly adjusted take here:
http://webtypography.net/Rhythm_and_Proportion/Horizontal_Motion/2.1.2/