The 'Days To Sell' data is retrieved from AutoTrader and if it's not showing, it's likely that you do not have the 'Full Retail Check' product with AutoTrader. 'Days To Sell' is AutoTrader's prediction of how long it will take to sell a vehicle. The 'Sold In' columns provide the actual time taken to sell a vehicle.
Note, different versions on the 'Sold In' column are provided for different dates such as 'Date Added' and 'Date For Sale', these columns can be enabled by selecting the 'Columns' button in the Vehicle -> Browse Vehicles section.