Since I've been here for two years, I have noticed that the way spammers irritate us has evolved from a lot of text-based stuff to relying on image-based spam. However, one thing that has remained fairly consistent is the amount of time a lot of spam runs last.
I remember in September 2004, after the Dan Rather-60 Minutes II incident, there was a lot of spam with the subject line "Dan Rather must be fired!" I predicted that the spam run would last less than 10 days, and sure enough, it lasted 9 days. We don't keep track of spam stats like that in general but for this one I did.
Nowadays, whenever we get a stock spam run there is a veritable snow storm of spam, however, like the old spam runs it usually lasts for about a week and then everything goes "quiet." This is not to say that there are not other spam runs going on simultaneously but these particular spam runs don't last forever. I have a couple of theories - since they are usually stock spams, the spammer has gotten out of the stock after a successful pump and dump and has no more reason to spam. The other theory is that the spambot network he is most likely renting probably was only available for a few days and his contract expired. In any case, these spam runs are constantly evolving so chasing them, as you all might expect, is a cat-and-mouse game.