You are looking for wp_insert_post_data
filter. In this function, to parameter is supplied for you to filter the data before it is inserted. You can use it like -
add_filter('wp_insert_post_data', 'mycustom_data_filter', 10, 2);
function mycustom_data_filter($filtered_data, $raw_data){
// do something with the data, ex
// For multiple occurence
$chunks = str_split($filtered_data['post_content'], 300);
$new_content = join('BREAK HERE', $chunks);
// For single occurence
$first_content = substr($filtered_data['post_content'], 0, 300);
$rest_content = substr($filtered_data['post_content'], 300);
$new_content = $first_content . 'BREAK HERE' . $rest_content;
// put into the data
$filtered_data['post_content'] = $new_content;
return $filtered_data;
}