c-guess-region-max is a variable defined in `cc-guess.el'.
Its value is

The maximum region size for examining indent information with `c-guess'.
It takes a long time to examine indent information from a large region;
this option helps you limit that time. `nil' means no limit.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.