Member Rara Avis
There aren't any hard and fast rules about the word count in a standard line, because it's going to be different for some topics and certainly different for various font sizes.
Think of it in terms of a newspaper. If you picked up your Sunday paper and the text stretched from the left side of the page all the way to the right side, you would quickly discover how hard on the eyes it is to read long lines of text. That's exactly why the newspaper and most magazines use columns - to help break up those long lines. Web designers have to be a little more creative (moving your eyes to the top of the "next" column is much more difficult on a web page, and it's not considered polite to ask your readers to scroll up-down-up-down to read a single article), but the goal is still the same. Make text easy to read.