Example of how to change a text string using the gettext hook.

pmpro-change-text-example-gettext.php

<?php
/**
 * This recipe is an example of how to change and translate localized text strings
 * for PMPro.
 *
 * 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 gettext_translate_per_text_domain( $translated, $text, $domain ) {

	switch ( $domain ) {
		case 'paid-memberships-pro':
			switch ( $text ) {
				case 'If your account is not activated within a few minutes, please contact the site owner.':
					// Your custom text string or translation here.
					$translated = 'Your text string here.';
					break;
			}
			break;
	}

	return $translated;
}
add_filter( 'gettext', 'gettext_translate_per_text_domain', 10, 3 );