Extend the subscription period intervals to a custom number

extend_sub_intervals.php

<?php
// Extend the subscription period intervals to a custom number
function eg_extend_subscription_period_intervals( $intervals ) {
	$max_intervals = 30; // The maximum number of intervals you want to display
	$intervals     = array( 1 => _x( 'every', 'period interval (eg "$10 _every_ 2 weeks")', 'woocommerce-subscriptions' ) );
	foreach ( range( 2, $max_intervals ) as $i ) {
		$intervals[ $i ] = sprintf( _x( 'every %s', 'period interval with ordinal number (e.g. "every 2nd"', 'woocommerce-subscriptions' ), WC_Subscriptions::append_numeral_suffix( $i ) );
	}
	return $intervals;
}
add_filter( 'woocommerce_subscription_period_interval_strings', 'eg_extend_subscription_period_intervals' );