Add an extra billing interval to WooCommerce Subscriptions

wcs_custom_frequency.php

<?php

function wcs_custom_frequency( $intervals ) {
    // change the $custom_frequency variable numerical value to what you'd like to add.
		$custom_frequency = 50;
		$intervals[$custom_frequency] = sprintf( __( 'every %s', 'my-text-domain' ), WC_Subscriptions::append_numeral_suffix( $custom_frequency ) );
		return $intervals;
}
add_filter( 'woocommerce_subscription_period_interval_strings', 'wcs_custom_frequency' );