How long will FFXIV be down for?

I think Beta starts sometime in December/January and release in March or maybe April.

I am optimistically hoping we get further clarification on the live producers letter.

Realistically, I figure summer (June/July) of 2013.

It should also be noted to the OP that the old version is going to be turned back on for general japery after the finale after some time being down, for players who do want to play around to do so without consequence to their characters.

As far as the solid date, you're going to get speculations everywhere. If you want to play it safe, expect next fall and then be pleasently suprised. IF you need a clearer idea as to what's ACTUALLY going to happen, wait for the official word.

A fairly long time.