Throw this in your child theme’s functions.php file
function graphene_custom_slider_excerpt_length(){
graphene_set_excerpt_length( 30 );
}
add_action( 'graphene_before_slider', 'graphene_custom_slider_excerpt_length' );
Change 30 to whatever number (words).
If your child theme functions.php file is empty, add <?php at the beginning of the code and close it with ?>