[Kessler83]

A vast majority on book typography agrees on 66 characters per line in one-column layouts and 45 characters per line in multi-column layouts as being the optimal numbers for reading. The text-block should also be placed assymetrically on the page, with the margins in size order being inner<top<outer<bottom. The line height should be set at 120% of the highest character hight for normal book typefaces, but should be increased for typewriter typefaces and can be decreased slightly with shorter lines. A small set of typefaces are economic without losing readability, and if you use them you can increase these numbers slightly. But any more than 80 characters and anything less than 40 characters is suboptimal for texts that are longer than a paragraph or so.

If you adhere to these very simple principles, you will have avoided like 95% of the typographic choices that can make texts hard or slow to read.

[OskarS]

Robert Bringhurst in "The Elements of Typographic Style" (a bible for this kind of thing, and one of the most beautiful books ever made), has this to say on the topic:

"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.

If the type is well set and printed, lines of 85 or 90 characters will pose no problem in discontinuous texts, such as bibliographies, or, with generous leading, in footnotes. But even with generous leading, a line that averages more than 75 or 80 characters is likely to be too long for continuous reading."

[5tefan]

This. Art of typography. I stick to this a lot. Major problem here: A lot of corporate templates suck and changing them is pita or impossible due to non negotiable guidelines.