The email is sent automatically the instant my server gets the notification from paypal that the transaction is complete. It usually only takes a minute or two.
Many ISPs implement email greylisting, which means they reject all emails and hope that the legitimate emails will be retried which my server is currently configured to do once every 3 hours.
If you don't get your key after 6 hours or so, there's probably something wrong and you should email me at support at blockland .us