Add the following snippet to your functions.php file or using the Code Snippets plugin:

// Extend Payment Plan lengths for 36 months, 48 months, 60 months, 72 months, 84 months
function wpp_extend_subscription_expiration_options( $subscription_lengths ) {

	$subscription_lengths['month'][36] = wcs_get_subscription_period_strings(36, 'month');

	$subscription_lengths['month'][48] = wcs_get_subscription_period_strings(48, 'month');

	$subscription_lengths['month'][60] = wcs_get_subscription_period_strings(60, 'month');

	$subscription_lengths['month'][72] = wcs_get_subscription_period_strings(72, 'month');

	$subscription_lengths['month'][84] = wcs_get_subscription_period_strings(84, 'month');

	return $subscription_lengths;
}

add_filter('woocommerce_subscription_lengths', 'wpp_extend_subscription_expiration_options');

After adding this snippet you will have the option to extend payment plan length for 36 months, 48 months, 60 months, 72 months, 84 months.