Yoshi stated in an interview that it should take about a month to transfer the data to the new server and that the beta and 1.0 would not be running concurrently. Here. http://ffxiv.gamerescape.com/2012/06...ks/#more-10676

The latest interview posted yesterday seems to indicate it'll be down for 3 months. Hopefully the old interview information is outdated and it will only be down during the open beta. Having FFXIV down for 3 months would be pretty harmful to the current community.

It would be nice to have some concrete information but they are probably unsure themselves while working against some tough deadlines.