My guess and this is just a ballpark, would be between 24-40 hours if they took all of your internal parts to install in a new housing.
I would look for a used rear end, then you are probably looking between 15-24 for the rear end swap. I would try a place like
http://www.worthingtonagparts.com/kubota-salvage or similar. You may have to pay freight to have one shipped to you or your dealer, but it may be cheaper than a "New" Rear end directly from Kubota.
Also, look on Craigs list in your area. There has to be some old kogger (if a reader he is one, my apologies) that spends his days buying, rebuilding/restoring old tractors and he may have one sitting in his back yard (or be able to help you source one).
Believe it or not there are people that work on tractor enough that they can tell you where to get any part for any tractor. They know all the tractor salvage yards across the country just like we know the auto salvage yards in our area. Start looking and calling, you will find something.