How long does the average spam outbreak last?

Ever since July, I have noticed that when spam outbreaks occur, they last for a couple of days and then subside.  There is a period of inactivity, and then they go again.  My question is this: how long does the average spam run last?

To calculate this, I referred to the data I have on our total message traffic. I simply scanned down the list of our total traffic and eye-balled occurrences where the total traffic was much larger than its surrounding partners.  Logically, this would correspond to a high-volume spam outbreak.  Since October 1, I have counted 18 such occurrences.

According to my calculations, the average spam burst lasts about 3.5 days.  They occur once every 8 days, which means that the next spam starts every 4.5 days after the last one finished, on average.  One day I’ll go back and trace the historical data to see if the frequency of spam runs are increasing, decreasing, or staying the same, and over what time periods do we expect the greatest amount of spam.