hook_cron_queue_info 'time' value I've been searching around, but I can not find a good explanation (or understanding) what the 'time' value in hook_cron_queue_info actually does, or how to set it to work properly.

From: https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_cron_queue_info/7

it is said that: 'time': (optional) How much time Drupal should spend on calling this worker in seconds. Defaults to 15.

I don't quite understand that. Does calling the worker mean time allowed to execute the cron queue job?
And does the next cron queue job starts after this time? Or when the previous job is completed?
It's literally what the description says it is - during a cron run, Drupal will spend a maximum of the seconds you provide in that value processing that particular queue, regardless of the number of jobs in it.

If you're looking for the relevant code it's in drupal_cron_run()

foreach ($queues as $queue_name =&gt; $info) {
  if (!empty($info['skip on cron'])) {
    // Do not run if queue wants to skip.
    continue;
  }
  $function = $info['worker callback'];
  $end = time() + (isset($info['time']) ? $info['time'] : 15);
  $queue = DrupalQueue::get($queue_name);
  while (time() &lt; $end &amp;&amp; ($item = $queue-&gt;claimItem())) {
    $function($item-&gt;data);
    $queue-&gt;deleteItem($item);
  }
}