Delay for sending out notification email

That highly depends on your website traffic. Mail sending is triggered by the visitors on your website. So if no one visits your site for two hours then the mails will not be sent. You can set the nervous finger to 0. Then it’s most likely that you trigger the sending and mails are sent out right away. I was already thinking of a way to trigger it with a cron job but most sites have enough traffic, that this is not an issue.

P.s.: I assume you are talking about the Post Notification plugin.