Function to add extra month intervals to the subscription editor page

woocommerce_subscription_period_interval_strings.php

<?php
/** 
 * Function to add extra month intervals to the subscription editor page
**/

add_filter('woocommerce_subscription_period_interval_strings', 'add_extra_month_intervals', 10, 1);
function add_extra_month_intervals($intervals){
	foreach ( range( 7, 12 ) as $i ) {
		$intervals[$i] = "every ".$i."th";
	}
	return $intervals;
}

?>