c-backslash-max-column

c-backslash-max-column is a variable defined in `cc-vars.el'.
Its value is

set-from-style

Documentation:
*Maximum alignment column for line continuation backslashes.
This is used by the functions that automatically insert or align the
line continuation backslashes in multiline macros. If any line in the
macro exceeds this column then the backslashes for the other lines
will be aligned at this column.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.