<?php

    /* Once in 10 seconds, 5 times per minute, 10 times per 5 minutes. */
    $rate_limits = array(10 => 1, 60 => 5, 300 => 10);
    $max_interval = max(array_keys($rate_limits)); /* 300 */

    /*
      For example:

      SELECT attempt_time FROM account_access_attempts
      WHERE account_id = ...
        AND attempt_time < NOW() - 300 seconds
      ORDER BY attempt_time DESC

      Or do a ZRANGE query to Redis, or whatever.

      Make sure access times are ordered, otherwise things will break.
    */
    # $attempt_times = get_account_access_times($username);
    $attempt_times = array(time() - 1, time() - 9, time() - 15);

    list($delay, $now, $count, $interval, $limit) = array(0, time(), 0, 0, 0);

    reset($rate_limits);
    foreach ($attempt_times as $ts) {
        $count++;

        if ($interval === 0 || $now - $ts > $interval) {
            list($interval, $limit) = each($rate_limits);
            if (!isset($interval)) { break; } # Safety measure
        }

        if ($count >= $limit) { $delay = max($delay, $interval - ($now - $ts)); }
    }
    unset($now, $count, $interval, $limit);

    echo $delay;

?>