Well if you are planning on installing from a Total War: Shogun 2 disc then you will in all likelihood be looking at roughly a nine gigabyte patch download, to patch your game up to compatibility and multiplayer integration with the Fall of the Samurai stand-alone expansion.
If you have a Fall of the Samurai disc then, I can only guesstimate, you would only be looking at a one gigabyte patch, at the worst. Since you would only need to download any patches that have come out post the release of Fall of the Samurai, since all other required data would presumably already be on the disc.
Hope that helps, and if you do decided to proceed then the wait will be worth it. Also, if you do not yet own the Fall of the Samurai expansion yet then when you do buy it you will not have to install much data, if any, since you will have already downloaded it in the previously stated massive patch for Total War: Shogun 2.
[This message has been edited by Scipii (edited 06-08-2012 @ 11:59 PM).]