I found the below study from Ticker Sense/Birinyi Associates of oil bull and bear markets dating back to 1986 enlightening.
What I find particularly interesting is how the bear markets are consistently down about a third, in terms of both the average decline and the median decline being around 30%. Of the past three bear markets (since 2003, when things really got kicking), two of the declines are nearly spot on 33% (-33.27%, and -34.47) and the other is -26.21%.
There is much less consistency in the rallies, where the average change is 62.89%, and the median is 37.30%, which basically tells us occasionally we get some huge rallies.
The latest rally dates to 1/18/07, has lasted 245 days, and we are up 62.32%.
Out of curiosity, I looked up my closest post to that date, which turned out to be 1/17/07, and was titled "Let the beatings, er, healings begin.". Not a bad day for that call!
Ticker Sense: Historical Bull and Bear Markets for Oil.