I think he is really asking how long it will take to attach.
But yeah once the attach (upload of file attachment to Gmail server is completed) the email will send instantly but it might take some time to reach its destination.
Also the last poster is correct. Gmail has a 10mb attachment size limit which the original poster probably figured out by now,
700MB attachment over email. That's crazy!
Just get a P2P software if you want to send files that size.