Here's what the site said when I sent it:
Quote:
Invitations are sent at the end of each day, after they are validated.
Please note that an invitation will not be sent if you entered an email address that is not a valid Battle.net account or a character name that does not exist.
So I'm guessing whenever the "end of the day" is. Given it's 12:45 AM where I am, but 11:45 where Blizzard's HQ is, I have no freaking clue.
I'd say wait a day, and if you still haven't gotten it, then something's probably wrong.