The Mail Now setting decides when the forum post goes out to the cron to then process the email notifications.
If a user does not check the Mail Now setting, then that user will have 30 minutes to edit the post before it is sent to cron to then email.
If a user does check the Mail Now setting, then the post will be sent to cron and the user does not have the option to edit the post anyway.
I believe how you have cron set up and how frequently you have it set up to run decides when the email notifications go out.