UPDATE: new version here:
https://bitcointalk.org/index.php?topic=610189.0Here is a list of all previous price drops of 15 percent or more. Peak, trough, number of days from peak to tough, and percent price drop.
1 2010-07-19 0.09 2010-07-24 0.05 5 44.4
2 2010-11-07 0.36 2010-12-10 0.19 33 47.2
3 2011-01-16 0.39 2011-01-19 0.31 3 20.5
4 2011-02-14 1.06 2011-04-05 0.67 50 36.8
5 2011-05-14 7.86 2011-05-21 5.97 7 24.0
6 2011-06-09 29.58 2011-11-18 2.14 156 92.8
7 2012-01-08 7.05 2012-02-16 4.19 39 40.6
8 2012-08-17 13.26 2012-08-19 9.09 2 31.4
9 2013-04-09 214.67 2013-04-16 65.33 7 69.6
The longest crash lasted 156 days. It has now been 153 days since the April 9 peak.
Typically, though, crashes are over very quickly. The median is only 7 days. The mean is 33.6 days.