These are cyclical lines which, based on the time frames from peak to peak and low to low, project equal time frames forward. Notice how the time frame for the total decline from 2007-2009 was about half of what it was from 2000-2002. Based on the future projections of the next implied "due" peaks and lows ( just assuming for the sake of argument that they will occur on such a "schedule" at all), the hypothetical time frame for the next decline will approach relatively no time.
Should a decline occur so rapidly at the implied point, I would not be surprised, considering the amount of speculative zeal over the years that has been encouraged by loose lending policies intended to maintain in the midst of a more powerful deflationary trend. The market has been prevented from naturally correcting for so many years that when it does fall under its own weight, it will presumably do so more forcefully and quickly than most can imagine.
Of course, at least three data points in agreement are necessary to propose the beginning of a pattern in anything, but in my humble opinion it certainly looks and feels like the equities market is fast approaching the third one.