Extend the expiration options for subscriptions

eg_extend_subscription_expiration_options.php

<?php

function eg_extend_subscription_expiration_options( $subscription_lengths ) { 
	$subscription_lengths['year'][10] = wcs_get_subscription_period_strings( 10, 'year' ); 
	$subscription_lengths['year'][20] = wcs_get_subscription_period_strings( 20, 'year' ); 
	$subscription_lengths['month'][48] = wcs_get_subscription_period_strings( 48, 'month' ); 
	return $subscription_lengths; 
} 

add_filter( 'woocommerce_subscription_lengths', 'eg_extend_subscription_expiration_options' );