This depends on the price of the unit, but here is the information as of December 2007.
0-199,999 # Actives 81 Days On Market 160 Months Supply of Inventory 8.10
200,000-349,999 # Actives 114 Days On Market 143 Months Supply of Inventory 8.44
350,000-499,999 # Actives 31 Days On Market 179 Months Supply of Inventory 12.40
500,000-749,999 # Actives 3 Days On Market 266 Months Supply of Inventory 36.00
Months Supply of Inventory means: it will take this long to sell all the units available that are currently on the market.
This also includes all the attached single family homes, townhomes, and condos.
Information from MLS of Northern IL and Heddrick-Wagner

The average list price is approx $363,000 but the average sale price is approx 332,000.