This snippet will allow you to add custom billing intervals / lengths in addition to the defaults included within the WooCommerce Subscriptions plugin.
function eg_extend_subscription_lengths( $subscription_lengths ) {
$subscription_lengths['week'][8] = wcs_get_subscription_period_strings( 8, 'week' );
return $subscription_lengths;
}
add_filter( 'woocommerce_subscription_lengths', 'eg_extend_subscription_lengths' );