Add a new renewal period to WooCommerce Subscriptions to allow for custom renewal periods for products. Specifically a new "every 7th" selection that will set a subscription to renew every 7th renewal period..

custom-renewal-period.php

<?php
/**
 * Plugin Name: WooCommerce Subscription Custom Renewal Period
 * Description: Add a new renewal period to WooCommerce Subscriptions to allow for custom renewal periods for products.
 * Author: Nick Green
 * Version: 1.0
 * License: GPL v3
 */
function wcs_custom_renewal_period( $subscription_periods ) {
    $subscription_periods[7] = sprintf( __( 'every %s', 'woocommerce-subscriptions' ), WC_Subscriptions::append_numeral_suffix( 7 ) );
    return $subscription_periods;
}
add_filter( 'woocommerce_subscription_period_interval_strings', 'wcs_custom_renewal_period' );

If you need to add more than one option, you can do it like so:

<?php
/**
* Plugin Name: WooCommerce Subscription Custom Renewal Period
* Description: Add a new renewal period to WooCommerce Subscriptions to allow for custom renewal periods for products.
* Author: Nick Green
* Version: 1.0
* License: GPL v3
*/
function wcs_custom_renewal_period( $subscription_periods ) {
  $new_periods = array( 20, 40, 80, 160, 320);
  foreach ($new_periods as $period) {
    $subscription_periods[$period] = sprintf( __( 'every %s', 'woocommerce-subscriptions' ), WC_Subscriptions::append_numeral_suffix( $period ) );
  }

  return $subscription_periods;
}
add_filter( 'woocommerce_subscription_period_interval_strings', 'wcs_custom_renewal_period' );