It lasted five days, because that’s what is in the “source code”. On some realms the mileage may vary. They explain the chaos very good in this posting:
Hello!
We wanted to provide a quick update on the status of the Zombie Plague and Scourge Invasion pre-patch events, since there’s a lot of confusion floating around about how they are supposed to work and when specific events are supposed to fire. This is unsurprising considering how complex these events are. Apologies in advance as this post is going to be a pretty long one, but we felt it was important to be really detailed here with what’s happened, what is yet to happen, and how this diffe…