according to the documentation, it is more correct to change the length of the excerpt using add_filter:
add_filter( 'excerpt_length', function(){
return 10;
});
you can make your own function based on this, like this:
add_filter( 'excerpt_length', 'new_excerpt', 10, 1);
function new_excerpt( $lenth ){
return $length;
});
…
apply_filters( 'excerpt_length', 150 );