I'd guess 2-3 weeks of strong effect and some more weeks of a somewhat smaller, fading, but significant one.
We still don't know whether it will give it a WW victory too, to do it Japanes gap must be smaller than 14,000.
Anyway, it must be recognized that MS knows how to boost sales in its home market, while Sony still has to find its way in Japan.
Edit:
It looks like price drop is doing XB360 much better than announcing getting FF XIII, MS will be surely digesting this and trying to elaborate some new strategy... Too bad information overloading hits also giants, and getting broadly different feed-backs in different parts of the world looks like it's confusing everybody bar Nintendo.