Adjust the default value of cookie days when manually adding a new affiliate.

my_pmpro_affiliate_default_cookie_duration.php

<?php
/**
 * Adjust the default value of cookie days when manually adding a new affiliate.
 * 
 * Change the value of $cookiedays to your preferred default number of days (integer). 
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 */
function my_pmpro_affiliate_default_cookie_duration( $cookiedays ) {
	$cookiedays = 90;
	return $cookiedays;
}
add_filter( 'pmpro_affiliate_default_cookie_duration', 'my_pmpro_affiliate_default_cookie_duration' );