Use this hook to change the AJAX  timeout limit from 2000 (2 seconds) to something else when checking and applying discount codes on the Membership Checkout page.

apply_filters("pmpro_ajax_timeout", 5000, "applydiscountcode");

Parameters

int (2000)

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/pages/checkout.php