Double-click the narrow vertical bar immediately to the right of the map.  The top section shows the currently effective expiration times.  Stations will begin to fade at 80 minutes and will disappear at 120 minutes (at least, I think I'm running the defaults).  The numbers in () are the applicable times for buddies or friends from your filter or any station that you are actively tracking in any window.

You can set these values in the APRSIS32.XML file if you want something other than the defaults.

<Stations.MinAge>80</Stations.MinAge>
<Stations.MaxAge>120</Stations.MaxAge>
<Stations.MaxAge.Buddy>0</Stations.MaxAge.Buddy>