As I mentioned in your other question,
Some where (I can't find it right now), I read that 72 characters per line was best for reading comprehension. When there was less than this the end of line moving your eye to the begging of the next line slowed you down, and when there was more than this following lines become more difficult, because you have to move your head which makes eye tracking more difficult.
Now I have not been approaching this from a e-reader perspective, but from web design, mostly because my e-reader has small screen, and computers are getting wide screens designed for movies not for reading comprehension.
So lets look at how the print guys do this. For my sample I am using a broadsheet, a tabloid, the complete Sherlock Holmes, a physics text, a math text, and half a dozen paperbacks of various qualities.
Both newspapers use columns about 40 characters wide (proportional font, full justification) but the broadsheet uses shorter columns and puts another story on the bottom of the page the physics text uses lines about 80 characters long with a wide margin for notes the math text uses three columns about 40 characters wide. Both textbooks have graphical elements which break the flow of the text.
The Sherlock has Two columns about forty characters wide. The paperbacks varied from 50 characters wide to 70 (that one had almost no margin.) Aside from the broadsheet no effort was made to limit column length other than the size of the page, so apparently although although there is a limit for how long a column should be it is large.