line-number-display-limit-width

line-number-display-limit-width is a variable defined in `C source code'.
Its value is

200

Documentation:
Maximum line width (in characters) for line number display.
If the average length of the lines near point is bigger than this, then the
line number may be omitted from the mode line.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.