hi-lock-file-patterns-range

hi-lock-file-patterns-range is a variable defined in `hi-lock.el'.
Its value is

10000

Documentation:
Limit of search in a buffer for hi-lock patterns.
When a file is visited and hi-lock mode is on, patterns starting
up to this limit are added to font-lock's patterns. See documentation
of functions `hi-lock-mode' and `hi-lock-find-patterns'.

You can customize this variable.