PixelKicks gave a good answer.
In theory you shouldn't see much, if any, change in rankings, but in practice, any time you move pages, there is some risk. I don't know if you can do a slow migration or not, but if you can, I'd suggest you do the following.
Start small:
Move a portion of the pages (maybe a couple hundred thousand for your big site) first, track their traffic and ranking and see if there is any impact. I'd probably watch the pages for 3-4 weeks just to make sure. If you see a drop, you may want to keep watching them longer to see if it comes back on its own. This should give you an indication of what to expect for the other pages.
Setup Canonical Tags First
If you are able, setup the new pages (or entire new site) and then add canonical tags to all the old pages, pointing to the new pages. This will actually start the transition process for Google and others and you may see the pages with canonical tags having been switched in Google in just a few days. After you have seen that most or all the pages have been switched in Google, then setup the 301 redirects.
You can see Rand talking about this process in the video on this page: http://moz.com/blog/cross-domain-canonical-the-new-301-whiteboard-friday
How Long to Recover:
You may not see any drop in rankings. You may see a little shifting for a few days, but then everything settles back to where it was. If you do see a drop, it's likely that it will recover after a few weeks or months (as PixelKicks indicated). If, on the other hand, you see a drop and it doesn't self-correct after several weeks, then the recovery is a matter of having to rebuild some authority to your site. The time that will take will depend on how much work you put in to doing that and what your competition is like.
Good luck. That's a huge site.
Kurt Steinbrueck
OurChurch.Com