But someone will always have to either scroll horizontally or wrap the text. I’m speaking as someone who often views code on my phone, with a ~40 characters wide screen.
In typography, it’s well accepted that an average of ~66 chars per line increases readability of bulk text, with the theory being that short lines require you to mentally «jump» to the beginning of the next line frequently which interrupts flow, but long lines make it harder to mentally keep track of where you are in each line. There is however a difference between newspapers and books, since shorter ~40-char columns allows rapid skimming by moving your eyes down a column instead of zigzagging through the text.
But I don’t think these numbers translate directly to code, which is usually written with most lines indented (on the left) and most lines shorter than the maximum (few statements are so long). Depending on language, I could easily imagine a line length of 100 leading to an average of ~66 chars per line.