Allow user to define a custom expiration interval (defined in constant)

pmpro_custom_expiration_interval.php

<?php
/*
Plugin Name: PMPro Custom Renew Button interval
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Customization to let admin specify how early to show the renew button.
Version: .1
Author: Thomas Sjolshagen @ Stranger Studios <thomas@eighty20results.com>
Author URI: https://eighty20results.com/thomas-sjolshagen/
*/

define('CRBI_DAYS_BEFORE_EXPIRY', 30);

function crbi_is_level_expiring_soon( $is_expiring, $level )
{
    if( !pmpro_isLevelExpiring( $level ) || empty( $level->enddate ) )
        return $is_expiring;

    switch ($level->expiration_period)
    {
        case 'Week':
            $expires = $level->expiration_number * WEEK_IN_SECONDS;
            break;

        case 'Month':
            $expires = $level->expiration_number * WEEK_IN_SECONDS * 4;
            break;

        case 'Year':
            $expires = $level->expiration_number * YEAR_IN_SECONDS;
            break;

        default:
            $expires = $level->expiration_number * DAY_IN_SECONDS;
    }

    if ( $expires < CRBI_DAYS_BEFORE_EXPIRY * DAY_IN_SECONDS )
        $expires = CRBI_DAYS_BEFORE_EXPIRY * DAY_IN_SECONDS;

    if ( (current_time('timestamp') + $expires) >= $level->enddate )
        $is_expiring = true;

    return $is_expiring;
}
add_filter('pmpro_is_level_expiring_soon', 'crbi_is_level_expiring_soon' );