I have a gtk TextView in a maximized window and I want to know how many characters a line can fit before you have to scroll.
(Friendly reminder, please stop putting answers in the comments, people.)
To summarize:
It depends on the size of the TextView, size of the window, size of the font, and as Alex Martelli said, the particular font and the usage of letters..."i" is a narrow letter, "m" is a wide letter, thus you can fit more "i"s than you can "m"s in a given space.
So, in short, there is no way to know for sure.
Get the length in characters of a PyGTK TextView