Look at the Case-Shiller index's bottom after the last housing bust in 1989-90 (as the 20-city index did not exist back then, we used the 10-city index). The index bottomed in September 1993 - more than two years after the U.S. economy had begun to recover - at a value of 75.81. Nominal gross domestic product (GDP) rose by 109% between the third quarter of 1993 and the third quarter of 2009.
However, the population rose by about 20%, so nominal GDP per capita rose by 74%. (Real GDP per capita rose by 27%, a pretty mangy performance over 16 years.) House prices can be expected to inflate about as fast as nominal GDP per capita, in a large country like the United States where space is not yet at a premium.
Thus the Case-Shiller Index this time around might be expected to bottom at 132 (75.81 x 174%). Its current value is 157, so we can expect a further 16% drop, even if you assume the bottom is no lower than after the milder housing downturn of 1989-90. That bottom will probably be reached around the end of 2011 if the 1990-93 post-recession pattern plays out.