Add a new billing interval to WooCommerce Subscriptions. Specifically a "every 25 weeks" billing interval to selling a subscription to something and be charged every 10 weeks.

extend-subscription-intervals.php

<?php
/**
 * Plugin Name: Extend WooCommerce Subscription Intervals
 * Description: Add a "every 25" billing interval to WooCommerce Subscriptions
 * Author: Brent Shepherd
 * Author URI: http://brent.io
 * Version: 1.0
 * License: GPL v2
 */

function eg_extend_subscription_period_intervals( $intervals ) {

	$intervals[25] = sprintf( __( 'every %s', 'my-text-domain' ), WC_Subscriptions::append_numeral_suffix( 25 ) );

	return $intervals;
}
add_filter( 'woocommerce_subscription_period_interval_strings', 'eg_extend_subscription_period_intervals' );